Since your question is "where is the CG", not "where should the CG be?" there's no need to get all worked up about mean effective (or aerodynamic) chords, etc. The CG is simply where chordwise the plane balances.

Hold the plane with a finger under each wing on each side of the fuselage such that the plane is level. That's where the CG is. Then use this point to establish that the step should be 0.5" back.

As mentioned, also make sure that the wing is 1 - 2 degrees positive incidence relative to the floats.