Inset corners -- 3' 90 deg radius x 4 => 3^2 * 3.14 * 3.2 * 7.46 = 700 gallon deduct
Steps: 1/2(4' inset * 2.8 deep) * 6 wide * 7.46 = 250 gallon add

Shallow part of pool: 20 * 40 * 3.2 * 7.46 = 19,098 gallons add
Deep part of pool: 20 * 20 * [(8.3 - 3.2)/2] * 7.46 = 7,610 gallons
[Probably should be [(8.3 - 3.2)/3] -- but it depends on the shape of the slope and hopper ]

19098 + 7610 + 250 - 700 = 26,258 total