Formula Results on Aggregated Values doesn't make sense

Hallo Experts,

I have a BEx Query with a Processstart Date and a Process End Date, which are keyfigures in my Cube.

What I would like calculate the Processing Time, which is Enddate - Startdate. Which is working correct.

Not all processes actually have an enddate that is why i calculate the processing time only for the ended ones.

In addition i would like to calculate an average processing time, enddate-startdate / number of processes.

I am trying to do calculate an Average Processing Time.

Is there a way to calculate the formula on the aggregated levels instead of summing, or whatever aggregation behavior was choosen?

Regards

Daniel