One thing you could try, its something I do form time to time.
If you know if your PT will not change size, i.e the columns and rows will not grow or reduce (I often use PT's for a summary display rather than for changing views of data) then you could add in the next column after the PT has finished your formula.
So lets say the last column of your PT is column E, now in column F, enter your formula as you want it.
I have explained that really badly, sorry but what I mean is that you do not have to use pivot fields/pivot items in order to run formula over PT's
Have a think and come back and I will try and post an example if it's not clear