Refer to other Summarize columns in a Custom Expression?

Hello there,

I need to create some % calculations (such as % of Active Users).
Mathematically to achieve that it is (# of Active Users / Total # of Users).

I tried creating two custom Expressions first in the Summarize area, and then attempted to use those both in a 3rd summarize but it does not allow me to refer the first two.

I am wondering as an alternative, if our admins create a two metrics that calculates (# of Active Users) and (Total # of Users) would I be able to refer to those metrics in a Summarize custom expression?

Thank you