like a lot of 'rules of thumb' they're good for thumbs and rules but not necessarily for the person. "100 degree rule,,",,,"50/50rule",,"length of paddle is from here to here"..etc. There is NO direct relationship between so many inches beam at max measurment and "a boat I can grow into". How the hull handles in the water is a LOT more than x (cm, in, m,ft, furlongs, cubits, etc.). Its about as possible to get caught up in dimensions as it is pushing this mouse around but they really don't get one as far as the paddle will.