On a trainer with a hershey-bar wing, the wing chord and the MAC are the same. The wing chord is how far it is from the leading edge to the trailing edge. So on a .40-size trainer that's usually about 10". The balance point is usually at 30%. pick up the plane with your fingertips under the spar. That's usually where the Balance point should be. <Scientific content alert> The plane balances on the spar because the spar is the strong part of the wing, and that is where the lift forces act through. If they put the spar somewhere else in the wing, when the wing lifted the plane the lift forces would twist the wing and it...would...be....bad. </Scientific content alert>

So with a 10" wing, expect it to balance at 2.5 to 3 inches, and put the step of the float at 4 inches.

If you have swept or tapered wings, search out "Sparky Paul" for a graphical method that works pretty well.

I think your problem is that you need more incidence. OR something weird, like you sanded the step on your floats round. If you have flaperons, droop them a little for takeoff.