What size overflow?

I have a 40 breeder and a large refugium about 50 gallons I believe, with 2 mag 7 pumps in refugium. (May only use one most likely) but my question is what size overflow should I use? I've researched and from what I can tell the overflow gph must exceed that of the mag pump (7 as in 700 but I read they only do 650 possibly) I just don't want to order the wrong overflow and have an actual overflow or my sump to run dry. Plus I want it to filter a good bit of water obviously haha. In case anyone's wondering, I had the sump on a 125 previously. But there's no waaaay I'm selling it ever because it's awesome, so I'm going to put it on my 40 breeder... Please help! Thanks :dance: