Help on reports

Hi!
I have a one query matrix. Now, I want to add a summary. My problem is that the summary should not be for the whole column. i.e.: this is my report

Enrollment                  F90      S91
100 Level (0-199)             5       7
200 Level (200-299)           5       8
Lower Level (0-299)           10      15
300 Level (300-399)           15      12
400 Level (400-499)           5       9
Upper Level (300-499)         20      21

Total                        30       36
                             ^^^^^^^^^^^^
                             I want the summary without including Lower 

I have tried many different things and nothing seems to work. The simplest thing to do would be a formula column where I sum everything but Lower and Upper Level. But when I position that formula in the cross product group I get an error message since I am referencing columns from a lower group.
Does anyone know how to do it?
Thanks in advance.

                     Andrea.