Use the CG that you like flying the plane at on wheels and use that position for reference. A 1/2" either way isn't going to hurt anything. Most critical is that the planes wing should be slightly positive to the top of the floats. The top of the floats is a datum for most floats. Be sure your struts are rigid as you don't want the floats flopping around on take-off.
Welcome to the fraternity,
Peter