Depends if you are talking about the amount of water passing through your wet dry filter media ( which it sounds like ) or total turnover of water circulation within the tank.
They are different things entirely.
Rule of thumb for filtration turnover is around 3-5 times the total tank volume in GPH
Rule of thumb for total tank circulation is around 5-10 times the total tank volume in GPH, which includes filtration turnover.
Of course these are just estimates and people design their tanks water flows however they want.
Gravity feed overflow - assuming you have total freefall of the water, with no restrictions in the line to the sump, would have to calculate it to be exact - but off hand I believe a 3/4" bulkhead fitting with 3/4" pipe ID, with a 22 inch vertical riser would normally pass about 400-500 GPH water.
If I had that set up I'd either increase the drilled hole diameter or use more of the 8 holes besides just the two. You could then be confident on using a higher rated pump, divert some of the water overflow to your wet/dry, divert another portion of the water from the other overflows to your sump to increase your tank circulation. Valve it all off underneath with a series of ball valves, like a manifold, that would allow you to make changes once installed and up/running.
In other words - don't limit yourself up front before you add your substrate and rock ..etc. Once the tanks filled - you don't want to find out you can't get enough flow back to your sump and wet/dry .. ya know.
