my sump is 8w x 21L x 18H = 13 gal
"ish" I keep water lvl @ 15" making it 10 gals. That leaves me 3 gallons of back flow from display.
The Fuge is 13" cube but 23 tall making it 16.5 gal
& I have over flow @ 20" into sump. That makes it a even 15 gallons with 1 gallon back flow room. In total I get 4 1/2 gallons that can come back from display in case something happen. I have not tested it yet but the level of my overflow in the main tank should cut off at about 4 gallons with the 90 on the bulkhead.
The only thing that sucks about my sump & refugium is that there is no taking them out. The skimmer and all other equipment will come out fine but the sump and refug had to be put in before the stand went against the wall, I also notched the back support to the stand to allow the refug to hang out of the back of the stand giving me just a little extra room. I guess all in all I happy enough with the setup. BUT the splashing & salt buildup on the Janky MDF stand is really starting to cause it to expand and I fear I'll come home o a floor full of rock and fish.