For most people’s pumps and tanks size 900-1200 GPH should be plenty, as they would need a pump to keep up with that flow.. Of course the overflow’s linear dimensions and tank size (H) all effect the head [pressure of the tank water which then affects the actual flow you can or need to get.

There is a calculater on Reef Central that tells you what GPH to expect with a given Overflow and Bulkhead size. But you work backward from the GPH you wish to get.... then it gives you the recommended Overflow size and pipe size

IE for 1200 GPH you have a 18" Linear Overflow which would need a 1.43" OD pipe or (1.5")

Here is the link.. Hope it helps.

http://reefcentral.com/calc/drain.php

Let me know if you have any more questions.. I will update my listings with some new info...

What size Overflow and Pipe do you need?