okay, lets just say that mkroher's 1" of drainage is a standard. that would mean that your 55 gallon tank would drain almost 4 gallons, so you need at least 4 "empty" gallons in your sump.
considering the dimentions of your sump tank, for every 1" in H, the tank holds roughly 1.5gallons.
---1"X 29" X 12" = 348 cubic inches / 231 = 1.50649351.
so, if your DT drains one inch of water ( ~ 4 gallons) then your sump tank will need about 3" of empty space. now this is assuming that your returns AND overflow box only drain 1" total of water from teh DT.
now, as far as the return chamber is concerned: technically it's longer than 6" because of how your baffle is set up, and i'll assume that's a 1" gap b/w the baffles making the return chamber 7". considering you have a maximum of 12" water possible, even though it would be less than that, i'll use the 12" water depth as a factor in the equasion. thats 7" X 12" X 12" = 1,008 cubic inches of water in return chamber. 1, 008 / 231= 4. 36363636
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gallons. (thats a very generous equasion since the water in your return chamber most likely wont be 12" deep) but since you will have the return drilled into the side of the tank, once the water level falls below the top of that bulkhead, your pump will be pushing air.