I think it is more important for the width of the overflow to be as wide as can be so that you are skimming to the maximum surface area. Depth should not be, but i am no expert what so ever. Just common sense i suppose. i guess maybe extra depth is some sort of fail safe that i am unaware off? but then again that is what the emergency drain is for. If you look at all the modular designs the depth is short as possible. Look at modular marine or exotic marine or synergy overflow as a sort of baseline.