Overflow and return pump matching


Reefing newb
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I will be setting up my sump in a couple weeks and I'm looking for a return pump to match up with the overflow box I have. Problem is the box is basically a DIY job so I'm not sure what kind of gph it's going to do or anything like that. The guy I got it from had a quiet one 4000 pump that's rated for over 1000 gph which I think is much more than needed for a 40 gallon tank/30 gallon sump. Anybody have any clue on how I can figure out how large a return pump I need to get the right kind of setup going?
I don't think that it would be too too much or anything. Because you can't forget that the pump will have to be going against gravity when it comes up. But I'm not too sure on the exact numbers, I read somewhere that going up about 3-4 ft reduces the amount by half.
I heard back from the guy who sold me the box. He had it on a 75 gallon tank and this same 30 gallon sump and he said he had that pump dialed back about half way with a ball valve so I'm guessing maybe about a 600 gph pump or around there would work.