refactor_me.rb

  VAT_RATE = 20.0

  # Public: Get the total amount for an invoice taking additional users in to
  # account.
  #
  # Params
  # ex_vat - if true, subtract 20% from amount
  #
  # Returns a number
  def total_amount(ex_vat = false)
    if number_of_permitted_users <= BASE_PERMITTED_USERS
      total = instance.subscription_amount
    else
      total = instance.subscription_amount + (number_of_permitted_users - BASE_PERMITTED_USERS) * ADDITIONAL_USERS_EACH
    end

    if ex_vat
      return total * (1 - (VAT_RATE / 100.0))
    else
      return total
    end
  end