Shoot for 1".......damn that sucks. so moral of the story is 1 foot deep then? lol
A foot of water out of a 180 is alot of water going back into the sump. Easiest way to make sure your good once you're up and running is to drill a small hole in your return(s) about 1/2-1" below the water line. That way, if or when you cut the power these "siphon breaks" will do just that..... Break any back siphoning into the sump.
I put swing check valves on my returns as well. It's a little overkill, but I've never had a flood in all my years of fishkeeping from backsiphoning.
Good luck and post some pics when you can!

