Calculated rows in a table

Hi , I need to build a matrix table with a couple of dimensions on it. This is the structure of the table :

I know that I can get line 3 with a dimension that groups lines 1 and 2 (same with line 6). But what about line 7 (which is the result of line 3 - line 6) ?

So my goal is to have a couple of calculated subtotals that sometimes grab different lines from the report. Which is the best way to achieve this?

Thanks