Spacing of floats. My floats are to be 15 inches apart. Is this measurement to be taken from the center line of each float or inside to inside?

I'd go with c/l to c/l, although the difference shouldn't matter much. Just make sure they'r absolutely parallel to each other. "Toe-in" will make them plow, and "Toe-out" will make the plane tend to be hard to steer and wobble from float to float.

I set my floats up with the centerlines the same distance from each other as the wheels would be if using conventional gear.

khodges, I'm a newbie to seaplanes so thanks for the help. Ok on c/l to c/l, no toe-in or out.

The etched-in-marshmallows rule of thumb for float spacing is 1/4 wingspan. I agree about parallel but there are those who will argue for toe-in.
(Marshmalllows are not quite as official as if the same rule were etched in granite)

Thanks Jim. 1/4 of the wing is 15 inches. I'm going to go with the floats being parallel and give them a try.