Overflow and Sump Question

[Baldy]

Im sorry if i post this in the wrong section, but it has to do with a sump im trying to design, and i figured it had to do with reef aquariums.

Im trying to stuff a sump into a very small aquarium stand. It is only 25" from the top of the stand to the floor, and with the bottom of the cabinet sitting 4" above the floor, and the door openings it doesnt leave much room. Ive got the size of the biggest tank i could fit in there, and i plan on building it myself out of acrylic (practice for a possible future aquarium =) ). I also plan on using a CPR overflow as my tank isnt drilled for an overflow.

Im wondering, with my 75g aquarium, how much spare room should i leave for if the power goes out? I know if the pump shuts off, the aquarium still flows until the water level is flush with the intake box on the overflow, but im wondering whats a safe volume to account for? 1 gal, 5 gal, 10???

any help would be appreciated