PDF Form totals and subtotals

I can calculate subtotals using the calculate javascript:

<calculate>

     <script contentType="application/x-javascript">this.rawValue = firstfield.rawValue + secondfield.rawValue;</script>

</calculate>

But when I try to add the subtotal fields like that to get a grand total, it doesn't work (the grand total field remains blank).  How do I get the Grand Total field to calculate the subtotal fields?