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:

function set_Discount()  {   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 = iten_count * 10 / 12      var ten_round = Math.round(ten_discount*100)/110      if (prive_level=='C10')      {    nlapiSetFieldValue('discountrate', ten_round, null, true);   }      else if (prive_level=='C5')      {    nlapiSetFieldValue('discountrate', five_round, null, true);   }  }