[Solved] Running Sum for ABC Analysis

Hi,

I'm trying to perform an ABC Analysis (e. g. "'A' customers are those that generate 50 % of revenue, 'B' customers the next 25 %, and 'C' customers the remaining 25 %).

In order to do so, I'm calculating the sales per customer, and sort the list descending by sales, that's simple. But how can I calculate the Running Sum on the sales values?

Example                                          
Thanks a lot for any advice!