Power Off DT water height

[Baldy]

Hello all,

Im crunching the numbers on a sump for my 75g. due to a short stand, i dont have much height to work with, so im trying to utilize all the space i have available.

My design thus far has room for 19g when its running, and 3" of space between the water level and the top edge of the acrylic. ive calculated that to stay 1" away from the top edge, i would have to have 4.29g flow from the display tank into the sump after the return pump is shut off, and i could have 6.43g flow before it starts overflowing.

this translates into 1-1/4" safe water height drop in the display tank, and my sump would overflow if it dropped more than 1-7/8".

I plan on running a 1200gph lifereef overflow (not willing to drill the tank at this time), and a mag12 return pump. so my question is this, how much can i expect the water to drop when the power to the return pump is shut off? has anyone ever measured their own DT?