A quick gist for a code demo

business-aviation.html

<div id="ctl11_ctl00_MainNavRepeater_ctl01_SubMenuPanel" class="subnav">
  
  <ul class="clearfix" style="text-align: left;">

    <li style="margin-left: 101px;"><a href="/business-aviation/featured-products">Featured Products</a></li>

    <li ><a href="/business-aviation/platforms">Platforms</a></li>

    <li ><a href="/business-aviation/transform-your-aircraft">Transform Your Aircraft</a></li>

  </ul>

</div>

main.js

// define browser width and subtract extra width from main nav     
var browserWidth = $(window).width();    
var mainNavWidth = $("#main_nav ul").innerWidth();
var extraWidth = (browserWidth - mainNavWidth)/2;

// find the active button left orientation
var activeButtonLeft = $("#main_nav li.active").position().left;
activeButtonLeft = activeButtonLeft - extraWidth;

// find the active button center orientation - to help center subnav under it
var activeButtonWidth = $("#main_nav li.active").width();
var activeButtonCenter = activeButtonLeft + (activeButtonWidth / 2);

// calculate the subnav width
var subNavWidth = 0;
$('#main_nav div.subnav ul li').each(function() {
    subNavWidth += $(this).outerWidth();
});

// determine sub nav center
var subNavLeft = activeButtonCenter - (subNavWidth/2);

// make adjustments to align subnav centered under active main nav button, or right or left if need be
if (activeButtonLeft == 0) {        
    $('#main_nav div.subnav ul').css("text-align", "left");
    $('#main_nav div.subnav ul').css("padding-left", "28px");      
} else if (activeButtonLeft > 750) {
    $('#main_nav div.subnav ul').css("text-align", "right");
    $('#main_nav div.subnav ul').css("padding-right", "50px")
} else {
    $('#main_nav div.subnav ul').css("text-align", "left");
    $('#main_nav div.subnav ul li:first-child').css("margin-left", subNavLeft);
}