Calculating % in group subtotal?

[nshefler]

I am having trouble calculating the percentage on the group subtotal as a percent of the grand total.

Example

REPORT XYZ
YTD Volume % of total YTD Volume
Line 1 100 10%
Line 2 300 30%
Subtotal 400 (How do I get this)
...
...
...
Grand total 1000 100%

Formula for the grand total in the report text box (in the Report Footer) is =Sum([YTD Volume])/Sum([YTD Volume])

Formula for the Lines (the Detail section) with in the groups is =[YTD Volume]/sum([YTD Volume])

I can't get the Group Subtotal lines (in the Group Footer) to calculate the correct %.

Please help, thanks.
NS