No need to get all wrapped up in the math. For these kindsa calculations, 2 decimal places tend to amount to spurious accuracy.
A gallon of fresh water weight 8 1/4 lbs close enuff. 125 x 8 = 1,000. Toss in 30 lbs to allow for the 1/4, and Bobs yer uncle. Then there's the 50 gallons aft. 1,000 dived by 5 is 200. 200 x 2 is 400. So what you hull
has to carry is the equivalent of five healthy lads forward, a two healthy lads and a small dog aft.
Don't we all carry the answers to those sorts of "good enuff" calculations in our heads, so we don't have to drag out the fancy new-fangled electronic abacuses every time we need such an answer :-)?