I have been flying floats since the early 70's and have tried several differant float to airframe ratios. While you can get away with floats that are in the 60% range, the key is how long the floats are from the step to the bow. If, when the step is mounted such that the model balance is right on the step and the bow of the floats is at least 1 inch in front of the prop, the model will probably fly ok although the model will be be difficult in rough water and the floats will probably need to have spray rails to the front inside of each float in order to operate. If the floats are any shorter than this length their is not enough floatation foreward of the CG. and the model will dig in on any take-off attempt and can trip on landing. I think that the float application you are planning is a little small and I would be looking at larger floats in the 44 to 46 inch range.