Instead of all the math that srm came up with, I suggest capsizing the boat on land and resting the masthead on a bathroom scale (Gently, you don't want to break the mast doing this.) Whatever the scale reads, divide it by 8 to figure out how many gallons you need for the float.

Remember, the float itself weighs something and since the float will be on the masthead, its full weight will have to be added to the scales reading before dividing by 8.