Problem with a calculated field in Pivot

Hi,

I am trying to calculate yield from a column (P/F) that has only 0s (Pass) and 1s (Fail)
In Pivot, I can easily make a field that has Sum(P/F) and Count(P/F). I have confirmed that Excel is counting the 0s and 1 correctly)

For yield, I would take (Count(P/F)-Sum(P/F))/Count(P/F) to get yield, but no matter how I try to do it within the Pivot (i tried using claucalted field) it doesnt come out with correct calcualtion and I am not sure why.

Any ideas?