For a bulkhead, 600gph sounds reasonable. For a siphon, its a bit less (until the pressure on the tank side increases). My 55g has a 1.25" overflow.
55g reef, 2x250W 10k MH
+ 2x96W PC
actninc, 10g sump, AquaC Urchin skimmer, 65lbs LR
xenia, mushrooms, wels. open brain, fungia radiata (orange), green eyed zoos
flame angel, mated pair of false perc, 6-line, firefish
20gH electric yellow cichlid