Magento rounding issue patch file

SUPEE-662_EE_1.12.0.2_v2.patch

diff --git app/code/core/Mage/Core/Model/Store.php app/code/core/Mage/Core/Model/Store.php
index 66a94f5..1a146a5 100644
--- app/code/core/Mage/Core/Model/Store.php
+++ app/code/core/Mage/Core/Model/Store.php
@@ -106,6 +106,12 @@ class Mage_Core_Model_Store extends Mage_Core_Model_Abstract
      */
     const MEDIA_REWRITE_SCRIPT            = 'get.php/';
 
+
+    /**
+     * Rounding Epsilon
+     */
+    const ROUNDING_EPSILON = 1e-7;
+
     /**
      * Cache flag
      *
@@ -941,7 +947,19 @@ class Mage_Core_Model_Store extends Mage_Core_Model_Abstract
      */
     public function roundPrice($price)
     {
-        return round($price, 2);
+        return $this->roundEpsilon($price, 6);
+    }
+
+    /**
+     * Round price through epsilon
+     *
+     * @param mixed $price
+     * @param int $precision
+     * @return double
+     */
+    public function roundEpsilon($price, $precision = 2)
+    {
+        return round($price + self::ROUNDING_EPSILON, (int) $precision);
     }
 
     /**
diff --git app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Shipping.php app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Shipping.php
index 358957d..375a5d3 100644
--- app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Shipping.php
+++ app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Shipping.php
@@ -63,7 +63,7 @@ class Mage_Sales_Model_Order_Creditmemo_Total_Shipping extends Mage_Sales_Model_
              * Shipping refund amount shouldn't cause errors, if it doesn't exceed that limit.
              * Note: ($x < $y + 0.0001) means ($x <= $y) for floats
              */
-            if ($baseShippingAmount < Mage::app()->getStore()->roundPrice($baseAllowedAmount) + 0.0001) {
+            if ($baseShippingAmount < Mage::app()->getStore()->roundEpsilon($baseAllowedAmount) + 0.0001) {
                 /*
                  * Shipping refund amount should be equated to allowed refund amount,
                  * if it exceeds that limit.
diff --git app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Tax.php app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Tax.php
index 30367ec..ba7949d 100644
--- app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Tax.php
+++ app/code/core/Mage/Sales/Model/Order/Creditmemo/Total/Tax.php
@@ -161,6 +161,9 @@ class Mage_Sales_Model_Order_Creditmemo_Total_Tax extends Mage_Sales_Model_Order
 
         $creditmemo->setGrandTotal($creditmemo->getGrandTotal() + $totalTax + $totalHiddenTax);
         $creditmemo->setBaseGrandTotal($creditmemo->getBaseGrandTotal() + $baseTotalTax + $baseTotalHiddenTax);
+
+        $creditmemo->setShippingAmount(Mage::app()->getStore()->roundEpsilon($creditmemo->getShippingAmount()));
+        $creditmemo->setBaseShippingAmount(Mage::app()->getStore()->roundEpsilon($creditmemo->getBaseShippingAmount()));
         return $this;
     }
 }
diff --git app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php
index d72dc17..38cfc00 100644
--- app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php
+++ app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php
@@ -324,8 +324,8 @@ class Mage_Tax_Model_Sales_Total_Quote_Subtotal extends Mage_Sales_Model_Quote_A
                 $baseTaxPrice   = $basePrice;
                 $taxSubtotal    = $subtotal;
                 $baseTaxSubtotal= $baseSubtotal;
-                $subtotal       = $this->_calculator->round($subtotal - $rowTax);
-                $baseSubtotal   = $this->_calculator->round($baseSubtotal - $baseRowTax);
+                $subtotal       = $subtotal - $this->_calculator->round($rowTax);
+                $baseSubtotal   = $baseSubtotal - $this->_calculator->round($baseRowTax);
                 $price          = $this->_calculator->round($subtotal/$qty);
                 $basePrice      = $this->_calculator->round($baseSubtotal/$qty);
                 if ($taxOnOrigPrice) {
@@ -340,8 +340,8 @@ class Mage_Tax_Model_Sales_Total_Quote_Subtotal extends Mage_Sales_Model_Quote_A
                 $storeRate      = $this->_calculator->getStoreRate($request, $this->_store);
                 $storeTax       = $this->_calculator->calcTaxAmount($subtotal, $storeRate, true, false);
                 $baseStoreTax   = $this->_calculator->calcTaxAmount($baseSubtotal, $storeRate, true, false);
-                $subtotal       = $this->_calculator->round($subtotal - $storeTax);
-                $baseSubtotal   = $this->_calculator->round($baseSubtotal - $baseStoreTax);
+                $subtotal       = $subtotal - $this->_calculator->round($storeTax);
+                $baseSubtotal   = $baseSubtotal - $this->_calculator->round($baseStoreTax);
                 $price          = $this->_calculator->round($subtotal/$qty);
                 $basePrice      = $this->_calculator->round($baseSubtotal/$qty);
 
@@ -352,8 +352,8 @@ class Mage_Tax_Model_Sales_Total_Quote_Subtotal extends Mage_Sales_Model_Quote_A
                 $taxPrice       = $this->_calculator->round($taxSubtotal/$qty);
                 $baseTaxPrice   = $this->_calculator->round($baseTaxSubtotal/$qty);
                 if ($taxOnOrigPrice) {
-                    $taxable        = $this->_calculator->round($origSubtotal - $storeTax);
-                    $baseTaxable    = $this->_calculator->round($baseOrigSubtotal - $baseStoreTax);
+                    $taxable        = $origSubtotal - $this->_calculator->round($storeTax);
+                    $baseTaxable    = $baseOrigSubtotal - $this->_calculator->round($baseStoreTax);
                 } else {
                     $taxable        = $subtotal;
                     $baseTaxable    = $baseSubtotal;
diff --git app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php
index b91ee46..92a13b5 100644
--- app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php
+++ app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php
@@ -376,23 +376,67 @@ class Mage_Tax_Model_Sales_Total_Quote_Tax extends Mage_Sales_Model_Quote_Addres
                 $baseUnitTax    = $this->_calculator->calcTaxAmount($basePrice, $rate, $inclTax);
                 break;
             case Mage_Tax_Model_Calculation::CALC_TAX_AFTER_DISCOUNT_ON_EXCL:
-            case Mage_Tax_Model_Calculation::CALC_TAX_AFTER_DISCOUNT_ON_INCL:
                 $discountAmount     = $item->getDiscountAmount() / $qty;
                 $baseDiscountAmount = $item->getBaseDiscountAmount() / $qty;
 
                 $unitTax = $this->_calculator->calcTaxAmount($price, $rate, $inclTax);
-                $discountRate = ($unitTax/$price) * 100;
-                $unitTaxDiscount = $this->_calculator->calcTaxAmount($discountAmount, $discountRate, $inclTax, false);
+                $unitTaxDiscount = $this->_calculator->calcTaxAmount($discountAmount, $rate, $inclTax, false);
                 $unitTax = max($unitTax - $unitTaxDiscount, 0);
                 $baseUnitTax = $this->_calculator->calcTaxAmount($basePrice, $rate, $inclTax);
-                $baseDiscountRate = ($baseUnitTax/$basePrice) * 100;
                 $baseUnitTaxDiscount = $this->_calculator
-                    ->calcTaxAmount($baseDiscountAmount, $baseDiscountRate, $inclTax, false);
+                    ->calcTaxAmount($baseDiscountAmount, $rate, $inclTax, false);
                 $baseUnitTax = max($baseUnitTax - $baseUnitTaxDiscount, 0);
 
                 if ($inclTax && $discountAmount > 0) {
-                    $hiddenTax      = $this->_calculator->calcTaxAmount($discountAmount, $rate, $inclTax, false);
-                    $baseHiddenTax  = $this->_calculator->calcTaxAmount($baseDiscountAmount, $rate, $inclTax, false);
+                    $amount = $unitTax ? $discountAmount : $price;
+                    $hiddenTax = $this->_calculator->calcTaxAmount($amount, $rate, $inclTax, false);
+
+                    $baseAmount = $baseUnitTax ? $baseDiscountAmount : $basePrice;
+                    $baseHiddenTax = $this->_calculator->calcTaxAmount($baseAmount, $rate, $inclTax, false);
+
+                    $this->_hiddenTaxes[] = array(
+                        'rate_key'   => $rateKey,
+                        'qty'        => $qty,
+                        'item'       => $item,
+                        'value'      => $hiddenTax,
+                        'base_value' => $baseHiddenTax,
+                        'incl_tax'   => $inclTax,
+                    );
+                } elseif ($discountAmount > $price) { // case with 100% discount on price incl. tax
+                    $hiddenTax      = $discountAmount - $price;
+                    $baseHiddenTax  = $baseDiscountAmount - $basePrice;
+                    $this->_hiddenTaxes[] = array(
+                        'rate_key'   => $rateKey,
+                        'qty'        => $qty,
+                        'item'       => $item,
+                        'value'      => $hiddenTax,
+                        'base_value' => $baseHiddenTax,
+                        'incl_tax'   => $inclTax,
+                    );
+                }
+                break;
+            case Mage_Tax_Model_Calculation::CALC_TAX_AFTER_DISCOUNT_ON_INCL:
+                $discountAmount     = $item->getDiscountAmount() / $qty;
+                $baseDiscountAmount = $item->getBaseDiscountAmount() / $qty;
+
+                $unitTax = $this->_calculator->calcTaxAmount(
+                    max($price - $discountAmount, 0),
+                    $rate,
+                    $inclTax
+                );
+                $baseUnitTax = $this->_calculator->calcTaxAmount(
+                    max($basePrice - $baseDiscountAmount, 0),
+                    $rate,
+                    $inclTax
+                );
+
+                if ($inclTax && $discountAmount > 0) {
+                    $amount = $unitTax ? $discountAmount : $price;
+                    $hiddenTax = $this->_calculator->calcTaxAmount($amount, $rate, $inclTax, false);
+
+                    $baseAmount = $baseUnitTax ? $baseDiscountAmount : $basePrice;
+                    $baseHiddenTax = $this->_calculator->calcTaxAmount($baseAmount, $rate, $inclTax, false);
+
                     $this->_hiddenTaxes[] = array(
                         'rate_key'   => $rateKey,
                         'qty'        => $qty,
@@ -457,8 +501,7 @@ class Mage_Tax_Model_Sales_Total_Quote_Tax extends Mage_Sales_Model_Quote_Addres
                     $child->setTaxRates($applied);
                 }
                 $this->_recalculateParent($item);
-            }
-            else {
+            } else {
                 $taxRateRequest->setProductClassId($item->getProduct()->getTaxClassId());
                 $rate = $this->_calculator->getRate($taxRateRequest);
                 $this->_calcRowTaxAmount($item, $rate);
@@ -524,8 +567,12 @@ class Mage_Tax_Model_Sales_Total_Quote_Tax extends Mage_Sales_Model_Quote_Addres
                     $inclTax
                 );
                 if ($inclTax && $discountAmount > 0) {
-                    $hiddenTax      = $this->_calculator->calcTaxAmount($discountAmount, $rate, $inclTax, false);
-                    $baseHiddenTax  = $this->_calculator->calcTaxAmount($baseDiscountAmount, $rate, $inclTax, false);
+                    $amount = $rowTax ? $discountAmount : $subtotal;
+                    $hiddenTax = $this->_calculator->calcTaxAmount($amount, $rate, $inclTax, false);
+
+                    $baseAmount = $rowTax ? $baseDiscountAmount : $baseSubtotal;
+                    $baseHiddenTax = $this->_calculator->calcTaxAmount($baseAmount, $rate, $inclTax, false);
+
                     $this->_hiddenTaxes[] = array(
                         'rate_key'   => $rateKey,
                         'qty'        => 1,
@@ -658,7 +705,7 @@ class Mage_Tax_Model_Sales_Total_Quote_Tax extends Mage_Sales_Model_Quote_Addres
                 $baseTaxSubtotal    = max($baseSubtotal - $baseDiscount, 0);
                 $rowTax             = $this->_calculator->calcTaxAmount($taxSubtotal, $rate, $inclTax, false);
                 $baseRowTax         = $this->_calculator->calcTaxAmount($baseTaxSubtotal, $rate, $inclTax, false);
-                if (!$item->getNoDiscount() && $item->getWeeeTaxApplied()) {
+                if (!$item->getNoDiscount() && $item->getWeeeTaxAppliedAmount()) {
                     $rowTaxBeforeDiscount = $this->_calculator->calcTaxAmount(
                         $subtotal,
                         $rate,
@@ -674,8 +721,12 @@ class Mage_Tax_Model_Sales_Total_Quote_Tax extends Mage_Sales_Model_Quote_Addres
                 }
 
                 if ($inclTax && $discount > 0) {
-                    $hiddenTax      = $this->_calculator->calcTaxAmount($discount, $rate, $inclTax, false);
-                    $baseHiddenTax  = $this->_calculator->calcTaxAmount($baseDiscount, $rate, $inclTax, false);
+                    $amount = $this->_calculator->round($rowTax) ? $discount : $subtotal;
+                    $hiddenTax = $this->_calculator->calcTaxAmount($amount, $rate, $inclTax, false);
+
+                    $baseAmount = $this->_calculator->round($baseRowTax) ? $baseDiscount : $baseSubtotal;
+                    $baseHiddenTax = $this->_calculator->calcTaxAmount($baseAmount, $rate, $inclTax, false);
+
                     $this->_hiddenTaxes[] = array(
                         'rate_key'   => $rateKey,
                         'qty'        => 1,

action in public_html directory with:

patch -p0 < SUPEE-662_EE_1.12.0.2_v2.patch

Discovered this introduces a rounding down issue on products i.e. 2x £2.50 = £4.99