Hey Ed,
The common rule is 75-80% of the length from the rudder post to the prop washer.(for dual floats)
This always seemed arbitrary to me.

Ed Westwood published an article in MAN in 1991 where he showed that the step should be at 40% of the MAC, the nose of the float should be 1/3 of the prop diameter ahead of the prop, and the step is in the center of the float. The float length would then be 2(d/3+prop to .4MAC). In most cases, this makes the floats a bit longer than 80%

If you're buying floats and the 75% rule lands between two sizes, pick the larger size.