Hi Corper,
My tank is a 62g and my sump is a 33g.
I like to have as much water in my system as possible and still be safe if the power goes out .... so, I shut down my return pump and let the tank drain down to the level of the overflow, allow all the plumbing to drain back into the sump, unplug my protein skimmer (in sump Euroreef).
After everthing has drained out (simulating a power outage) I adjust that water level to about 3 inches from the top of my sump.
I have never had a problem, and it is good to know that if the power ever goes out or my return pump quits the worst that can can happen is I lose flow back to the tank
