as fredsedno stated, 25% is what i learned as well.

      one thing to keep in mind, as they get wider, there is more chance for ground looping, espescially on grass, because the width from wheel to wheel will act as a lever against  forward inertia and try to stop the plane  when a wheel on one side suddenly hits a bump or rolls into an abrupt depression .  wider may look better, but cheating on the narrow side will roll out better/straighter. and if you're on grass keep the wheels big...3 inches on a 40-50 size plane ain't too big.