I've been using the trial and "almost" error method on the sump so far since it's been a temporary effort.Ahhh I see, I misinterpreted the need for more sump volume. I was thinking it was for more filtration. Have you calculated where you'd like the height to be in the tank and how much water you need to be able to hold if the power goes out?
Depending on how much that is the 500 gallon might have to be run at half it's height or you could have a satellite bin that is attached to the sump via an emergency drain (or 2) to handle all the water.
I ran two Hammerhead golds on the last system with the 200 gallon sump and it contained all the back flow with the holes in the return pipes to break the syphon so I knew it would work.
If/when I add additional turnover to the tank I will need to be a little more strategic in what the sump can hold.
I have some time this weekend to work on the tank but the sump situation is still up in the air...
I may start adding some more fish and clean coral rocks to the big tank from the 240. looking to take that down as a first step. We will see