Reporting Services -- Aggregate Row and Col groups simultaneously.

It seems like this should be easy.

I have a report that looks like this:

                                                           Col1   Col2   Col3
RowGroup1   RowSubGroup A           2         3       1
                       RowSubGroup B           4         2       1
                       RowSubGroup C           1         2       3
                       Total                               7         7       5

RowGroup2   RowSubGroup A           1         2       3
                       RowSubGroup B           4         7       1
                       RowSubGroup C           2         1       0
                       Total                               7         10     4
                       

I'm using the matrix control as the number of rows and columns can vary.  the numbers in th middle are actually summations of the raw data being returned by the query.

I want to turn the summations into percentages (of the subtotal on the row group).  I can't find a scope that works to get me the total that you see at the bottom of RowGroup1 or RowGroup2.

if I use sum(Field,"RowGroup") (naming the highest level row group, i get the sum including all of the columns.  If I use the column group name as the scope, I get the whole page of rows -- not just the specific row group that I'm looking for.

There has to be an easy way to do this, but I haven't figured it out yet.