On your installation You want the airplane to balance at the same location on the floats as it does when it flys on wheels.

That CG you already know on wheels so after the struts are made up and the floats are mounted as per above with the CG/step relationship it will no longer balance at that point and most often it is tail heavy a little. Manny times a few ounces of lead in the bows of the floats are just the trick. This is about the only time I ever suggest adding lead but doing it this way you will never have to rebalance the airplane agine when changing between wheels or floats. Don,t add any lead to the airplane only to the floats.

Many of the fellows tend to make the float spread a bit to narrow so turning back downwind the airplanes tend to get easily turned over. I would suggest the float spread be at least one quarter of the wingspan on centers. Note this is a little more than the average spread of wheeled landing gear.

John