CASE count in relaton to selected Business Case

Hello dear community,

I`m sure you kann help me.
I’ve have an Olap table, in the dimension is the table, which shows certain change activities.
As KPI I want to calculat a relation of case count(of change actvities)/case count (dependant of different business cases)
In the denominator should be a variable, that shows the case count of different business cases.

Thank you, I hope you understood me