Questions - Need some input

Good Morning Reef Addicts,

I'm trying to figure out what size return pump is needed for my 120 gallon tank. I have a 5 foot delivery head on a 120 gallon tank with 2 mega overflows (Oceanic Ultimate Series), which are rated at 600gph each. I found a blurb on Marc's site that I should have a return gph pump of 3x to 5x the display tank. The Eheim 1260 has a 635 gph rating but at 5 feet of delivery head it brings it down to about 400gph. This makes it about 3.33x the display tank. But ...