best practices on building epm report containing 2 reports

hi bpc experts:

I m building a epm report which has to show data of 2 different models, the layout of the report is as follows:

As you can see the third table is just a excel table which has a formula to subtract amount or Report1 and Report2, but what happens is that Report1 and Report2 can grow in the number of rows (both show the same values in accounts), so my excel table will crash since it has harcodes excel formulas...is there a way to accomplish this in a practical way to avoid harcoded formulas on the "excel table" ?

regards

mah