The only time I ever saw the float placement stated by a recognized authority and in a numerical format, he said to put the step at 40% of the MAC, which would be slightly behind the balance point. Other sources say to put it at the balance point.

Essentially, the floats are like landing gear. Taildraggers have the wheels a bit in front of the balance point, nosedraggers (tricycle) have the wheels a bit behind the balance point.

My suggestion: Start with the balance point over the step. If you experience lateral instability on the takeoff run, cheat it back a bit. If the floats want to plow and the prop gets a lot of water, go forward a little. Just keep the floats the same.

The airplane should float with the nose up a bit, and even with the floats a bit nose-up. THe tail of the float should not be submerged.