scope question for a sum

[jymm]

I have a matrix where I have multiple Row AND column groups

the rows are grouped by Quarter & Month. The Columns by product and packaging.

What I want is something like this:

Product A
Bags Bulk
Q1 Jan 100 200
33% 66%
Feb 300 400
43% 57%

the problem is getting the scope for the sum of the denomonator of the percentage. I do a

=Sum(Fields!Quantity.Value)/Sum(Fields!Quantity.Value,"Product")

(I know - watch for 0 divide - no problem there - just keeping it simple right now)

and for Jan I get 1,000 (the total for the whole Product regarless of month) and not 300 like I want.

so - how do I tell ssrs sum to look at both a column group AND the row group when doing the sum?

Thanks in advance