Matrix Row Total line calculation

Hi I have a hopefully simple question within the Matrix Total Calculation.

Have: Total Revenue and Total Expense something like below:

Revenue    Amount

   GL1

   GL2

   GL3

   Total     -----  (Total Revenue Subtotal)

Expense

   GL4

   GL5

   Total   ----- (Total Expense Subtotal)

Total   (Grand Total)

The last line, the grand total, sums Total Revenue and Total Expense, which is incorrect. 

How to redo this for just the last line to be 'Total Revenue' - Total Expense' ? Subtotals for revenue and expenses work fine, just unsure how to only change that very last grand total line

Many thanks for reading