Just how far back is acceptable for the step to be behind the CG ??

Thanks in advance for any help.

The farther back the step is, the more difficult it will be for you to rotate for takeoff.

There's not a magic number where if you exceed xx% you won't fly ever, but if you're in front of it you'll be fine.

Also, if your step is aft but you rig the nose of the plane a little high, you have a better shot at getting away with it.

All that said, I would be very suspicious of any setup with the step aft of the 50% of chord line.

Just finished and flew a Heinkel 31 on floats. Before I relocated the floats forward so that the steps are about 1/4" in front of the CG, it would not come up on plane. Now, it pops up in about 10 feet. (2.5 pound model with a Magnum 15) There's more to it than just step vs CG, but having the CG slightly aft of the steps is one way to easily avoid problems.

Rule of thumb - Don't go more than 1/2" behind CG

Ditto on what MinnFlyer said, also, the nose of the floats should be in front of the prop buy at least 2".