How deep for the sump?

I took care of some issues with my overflow box, made some repairs and in this pic all I done so far, took care of the primer issue, later today I will go get a used 20g deep for $10, monday i'll get some pexi-glass cut and i am wondering how deep should that sump be?

 

You should have the water level low enough to handle back flow from your tank without overflowing should power go out. You will need to do test power outages to find the right spot.

 

So what you mean is, say the pump quit working it should be low enough to handle what ever the overflow box drains until the water in the main tank stops draining?

 

another question I have is the flowbox is rated at 800gph with a 1' opening in the bulkhead, with the hardware I added to make it work the opening is now 3/4' would that mean the gph should be 600?