If you set the bottom of a Clark-Y airfoil or similar, flat bottom airfoil, at zero degrees, the actual incidence, measured from the leading edge to the trailing edge, will be 2-3 degres. This is more than likely the reason it was indicated to make the bottom of the wing parallel with the top of the floats.

For a symmetrical airfoil, you'll need to use an incidence meter or measure and calculate the degrees.