sum up of several percentage numbers in pivot

I have a percentage calculated with A divided by B, as an example, in data sheet and displayed in pivot table, for several weeks.
When choosing total (all weeks) in pivot all percentage numbers are simply summed upp. I want to see an average percentage for all weeks, using all A divided by all B.

How should I do this?
Thanks in advance!

It is not the average number of all percentage numbers I wanna see but sum of D-column divided by sum of C-column when choosing all weeks.