Pivot Table - Calculated Field on two groups of columns

Dear all

merry xmas -

I wonder if someone knows how i can go about this calculated fireld.

I have a bunch of columns for accounts. I have grouped them into two groups (income and expenses). What I want to do is add a calculated column to the pivot so I can expand the groups to see the detail of what makes up the group totals.

So I want to add a calculated column which takes "Group1" Income - "Group 2" Expenses = Margin. Then I want to add another column to take this calculated value of "Margin" and then add a column for "Margin"/"Group1 Income".

Is this doable in pivot tables?

Not sure if you need this but my products are in the rows and amounts are as values.

Thanks for any help you can give.