Milage Calculation

Hello,
I'm attempting to get my expense report to automatically calculate the amount for milage traveled.  I have my calcMilage() function set to the Post Sourcing Function on the custom code tab of my Expense form.  It doesn't seem like the function is firing when the milage category is picked (id15).  Here is my code (I borrowed most of it from another member on these forums):

 function calcMilage()  {   var CategoryValue = nlapiGetCurrentLineItemValue('expense','category');   if (CategoryValue == 15)   {    var rate = .52; //current milage rate       var sOdo = prompt("Please enter the starting odometer reading.","");     var eOdo = prompt("Please enter the ending odometer reading.","");    var xTotal = (eOdo-sOdo) * rate; //calculation    nlapiSetCurrentLineItemValue('expense','amount',xTotal); //sets amount    nlapiSetCurrentLineItemValue('expense','custcol12',sOdo);     nlapiSetCurrentLineItemValue('expense','custcol13',eOdo);    return true;   }  }