How much flow from a cpr cs50

I was just wondering how much flow i will have after gravity affects the flow of my overflow. The overflow is rated at 300gph and the is set right under four feet from overflow to the sump. I have a mag 5 pump which is rated at 500gph for a return pump and i have a tee in the line which is returning back into the sump with a ball valve to adjust the flow of the return. What is the best way to dial it in right so my tank doesnt drain or overflow?

 

I would close the valve on your return pump at least half way and watch the water level in the tank closely after you turn the pump on, slowly open the valve until the water level is just right for the overflow box and the water level doesn't keep rising. Just start out cautiously by closing it more than you'll need to.

Also just to clarify, what affects your flow down your overflow is the power of the pump, you don't have to worry about gravity affecting the flow down the drain/overflow pipe. It will drain as much as you pump into the tank up to its maximum amount.