how to code a percent change on a group summary?

I am reporting for a bank on deposit history.  I have the total deposits broken into the following groups:

Region
Deposit Group (ie ck, sav, cd, etc)
Date (I have selected records for end of month)

Total deposits are reported for each end of month period for each deposit group for each region by using a running summary on the detail level, which is the month end account balance of each account.  I would like to calculate a percent change on each period to graph.  

A previous post on reporting percent change gave the following code:

shared numberVar LineCount;
LineCount := LineCount + 1;
If LineCount = 1 then
   0
Else
   (
   if Previous({fieldname}) = 0 then
      0
   else
      ({fieldname}- Previous{fieldname})) / Previous({fieldname})
   )

This code works if it is a database record, but it doesn't like the summary operation I am using.  It reports:

"This Field has no previous or next value."

Thanks for any assistance!