overflow flow

[outtafocus]

Is there any way to calculate approx how much flow my overflow will produce?
I am using a Hoffer gurgle buster type overflow piping, with 1" pvc passing through the bottom of my tank and dropping straight down into my sump. I have no elbows or reductions.
I am trying to decide if a Mag7 will be enough for a return pump, I dont want to overdrive my overflow. The tank is only a 25gal cube so I am not going for huge flow.