Overflow rate?

[DJKoop]

I am setting up a new 90 gal tank and since it has a tempered glass bottom I had an overflow made on the back corner with a 1.5 in. ID fitting. Now what I am wondering is how much water that should be able to handle. I know it will be enough to handle sump run by a mag 7 pump. What I was thinking of doing is putting in a T and include a closed loop system. I was wondering if there is a formula for calculating how much flow I can add?