Calculation on Pivot table running totals
 

I've got an pivot table that shows the following:
First row is dates in ascending order
Second row shows a running total of sales $ by date
Third row shows a running total of gross profit $ by date
The first column is a list of stores

What I need, and can't seem to find a way to do, is show a running Gross
Profit percent by day for each location. The formula is GP%=GP$/Sales. I
can calculate a GP% on the table, but what shows as a % is a Daily GP%. I
can't seem to show a RUNNING GP% based on the information on the pivot table.

I pull the data out of Access straight into the table, so I can't calculate
outside the table first. Also, the results have to be easy enough that
others can show only the stores they want in the list.
--
Thank you for the help,

Dave