leaf level calculation for weighted average

I have two measures: "A_PCT" - a percentage value and "B"

The current aggregation function for A_PCT and B is SUM.

How can I achieve, that the following formula is calculated at leaf level (or for each dimension value): A_PCT * B and gets summed up afterwards.

When I write the formula A_PCT*B it is caluclating actually SUM(A_PCT)*SUM(B), which makes no sense because A_PCT is a percentage.

I need that for a weighted average.

I am using WebI 4.0 SP6 btw.

Thanks!