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[BCOrchidGuy]

Arty it's of my opinion that you can make it work the way it is. You're going to be limited by the single hole you want to use but you just have to compensate in other ways. Say your 5/8s hole will drain 500 gallons/hour (I have no idea if it will) then just make sure your return pump matches that, for tank circulation use powerheads etc. Your sump doesn't have to handle the complete water volume of your tank 10-20 times an hour, but you still want that turbulent water movment in your tank.
Clinton gave me a link and I'll share it with you, you can calculate your drain size and overflow size off this calculator.
http://www.reefcentral.com/calc/

Doug