Help with promocode scripting

As suggested by Yang in this thread: https://usergroup.netsuite.com/users/showthread.php?p=91776 I am reposting this here...

Can I script a promocode's rate, or simply the checkout discount rate?

I'm trying to vary the discount based upon the number of items in a cart. It also depends on the customer price level.

Reason: Customers prefer to see a physical discount at the checkout rather than a reduced price. I've been after better promocode functionality since we started, but nearly 3 years on still the same old... so I thought I'd try it myself

Here is my script:

<script language="javascript" type="text/javascript">  var price_level = '<%=getCurrentAttribute('customer','pricelevel')%>';   var item_count = '<%=getCartItemCount()%>';   var five_discount = item_count * 5 / 12      var five_round = Math.round(five_discount*100)/110      var ten_discount = item_count * 10 / 12      var ten_round = Math.round(ten_discount*100)/110      if (price_level=='Corporate10')      {    document.forms['checkout'].elements['discounttotal'].value = ten_round;   }      else if (price_level=='Corporate5')      {    document.forms['checkout'].elements['discounttotal'].value = five_round;   }  </script>