% calculation instead of subtotals

Hi,

I have a Crystal Reports where we displaying subtotal on a certain dimension. The Report is a crosstab Report. Now there in a KPI that is GM% which is like the division of 2 KPIs and then multiplied by hundred. Now the subtotal is showing the sum of all the % values but we want to display the (sum(KPI1)/sum(KPI 2))*100. Any idea how to achieve this?