New Tank & Refugium Questions HELP

Hello everyone I just bought a new 90 gal tank, the LFS said they will take my 90 and let me pay the diff for a 110. The 110 still will fit the stand and I like it cause it will be taller.
I was given a 40 gallon tank and I built a refugium out of it. I think I made the glass seperations too high. My question is how much water Overflow and pump return will I get on a 110 if I loose power to the pump? I have about 3 1/2" inches of free space from the top when the refugium has water in it = About 4Gal's of free space according to the vol calc.

What do you think? I feel I'm cutting it too close.

Here is a pic of the custom built refugium.

 

Taller also means you'll need better lighting...

 

It is cutting it close if you only have that much gallonage to spare. I have my system set for absolute minimum drainage in case of power outage and off the top of my head I think it measures to about three gallons or a little more (water surface area of 48x18 in a 75g tank) and one inch PVC. My hole is actually drilled just at water line in the return hose.