Access 2003 Running Sum Question

I have a report and sub-report that have running sums, one calculating
Material In, the other calculating Material Out. I added a text box to
calculate the inventory on hand (=Material In - Material Out). It works, but
it pulls the first running sum calculation, not the last. It looks somewhat
like below. Is there a way to get the Mat'l. Out Running sum to pull the
latest amount by date or something? Any help is greatly appreciated.
THANKS!!!!

Material In
4/4/10 250 ft Running Sum Mat'l. In
250 ft
Material Out Running Sum Mat'l. Out
4/7/10 125 ft 125 ft
4/10/10 100 ft 225 ft


Amount in Stock

125 ft (Should be 25 ft)