ARR SUM of distinct customers

Hi community! 

I have what feels like a very basic question, but I can’t seem to solve it.

In one of our dashboards we have a very simple report that shows the total timeline activities and distinct customers of said activities per month

This works perfectly fine. However, I was asked if we could have another that shows the ARR of said accounts, so, the SUM of the ARR from the distinct accounts.

Is there a way to do this within the report builder? I’m guessing I would need to calculate this via rules engine and to a auxiliary table on the MDA. 

Any thoughts?