Most people suggest a total turn over rate of between 10 - 2
tank capacity per hour. I usually shoot for about half that coming from the main pump and half from power heads inside the main display. Fish only tanks don't usually require as high of flow and are closer to the 1
side. Tanks with a large amount of SPS
and other coral which require high flow (since they are usually found in shallow reefs where the tidal currents and wave motion are high) are going closer to the 2
So the first step for you is going to be to decide what you think you would like to keep in your tank and start planning from there. I would suggest shooting for 450-650 gph
from your main pump. Be sure to take into consideration the head loss of your system. This is usually estimated by the pump manufacturer as a function of height.
Lastly, once you know what the flow from the main pump will be you can size your overflow and plumping accordingly. Be sure that you have some wiggle room in this factor as a "just in case". What i mean by that is if your pump puts out 650 gph
make sure that your overflows can handle at least 750.