LF: Some advice/imput on overflow design

[ScubaSteve]

I am days away from ordering the glass for my new build and I am firming up my final design. The tank will be 24"x24"x20" (50G cube) rimless with a rear external overflow via a 12" long rectangular notch cut in the back glass. I am debating, at the moment, how deep that notch should be.

I'm going to think out loud here, so bare with me a bit. I'd love to have people's input on this.

I've designed the tank to have a 12" straight cut notch with no teeth. According to "rules of thumb", this should be able to handle up to 800 gph without any issues (I'll be running a beananimal-style overflow with three 1" pipes for overkill redundancy). I plan on running a Mag 7 return pump with a throttling valve; with all the losses in the piping, this should give me about 500 gph on the return but I'll have the valve there to tune it back if needs be.

If I treat the notch as a rectangular contracted weir, at 500 gph the film thickness going over the edge of the weir will be about 0.35". If for whatever reason the power cut out, 0.87 gallons of water would drain to the sump, neglecting the amount in the overflow box and drain pipes (manageable). If for whatever reason the pump is giving me 800 gph (the maximum I'd want to run on that over flow), the film thickness over the weir would be about 0.5"; meaning 1.25 gallons of water would drain in the event of a power failure (still manageable).

If I made the notch 1" deep, at normal operation (500 gph) this would give me .65", or 1.62 gallons, of headroom before the tank overflowed if for whatever reason all three drains managed to plug up (ignoring the small amount of space in the overflow box). If I chose to run the return closer to 800 gph, I'd have 0.5' of head space.

So to throw a few questions out there....

-Does any of this make sense? Would someone recommend otherwise?
-Am I missing any areas where I might get overflowing tank or sump? (Yes, I know I have ignored the volume of water in the return pipes for this discussion)
-Comments?