Have you tried using Window_Avg(COUNTD(CustomerID))?
WINDOW_AVG is a table calculation, so if you are not familiar with those, you might want to search the knowledge base as they can be a bit tricky at times.
I've attached a workbook that demonstrates how it might be used. Please let me know if you have any questions or if you are looking for something slightly different.
Average of Distinct.twbx.zip 61.2 KB