I use a Mag 12 for a return on my 60 gal. with slightly more head loss than you are calculating with. I also have 2 Aquaglobe mini powerheads pushing 300gph each. Corals and fish are all very happy
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I would probably go with a Mag 12 or even an 18 on your 72 depending on how many return outlets you want from the pump.
HTH