Hi James,
Wish I could give you a good solid answer, but I can't.....
I guess a lot depends on what part of the country you're in , or more so what the density altitude is. I sometimes pull fuel out of our PA 61s using a 6 oz. tank. (??) We have three of them, with a good bit of time on them, but no where near being used up. NC, 300-600 feet ABSL, usually we have high humidity and fairly hot...... even with 10% nitro, 6 oz. has always been plenty.
??