RunningSum according to sales value

Hello,

I'm using Webi over Bex. Inside Webi I can use ranking in order to get my TOP 80% of customers according to sales (Ranking --> Top 80 % + Cumulative Percentage).

My problem Is that I want to create a formula that does the same, in order use count of how many customers like that I have and show it inside the report.

I've tried using "RunningSum" over my sales measure, but my problem is that RunningSum doesn't seem to start from the top - no matter what.

Any chance there's a way to force "RunningSum" To start from the customer with the highest sales?

I know there's a quick solution in SQL, but again, I'm using BW.

Thanks in advance,

Or.