I'm confused - you're pumping into your wet/dry? I hope not. What if the return pump craps out on you?
If you want to know how much it pumps, fill a 5 gallon bucket, or even better - 40 gallon drum(or fishtank for that matter. Any known volume will do. The bigger, the more accurate.), and time how long it takes to empty it. Should take 1/10th of an hour, or 6 mins to empty the drum at 400gph.