All Thumbs:

Scale accuracy aside, your method will give you the weight of the floats, minus the landing gear you took off. The gear can weigh as much as 12 oz. depending on the wheels used.

Your method is the correct one to figure out how much weight you are actually adding to the airplane, but it is misleading when trying to compare the weight of various float setups.

If your airplane is now 1 1/2 pounds heavier, I'd guess that the floats, by themselves, weight 2 pounds plus.

Jim