Thats only a rule of thumb. There is often more. The "norm" is about 15mm to allow water to flow in and out of the tube. Less may cause problems with cutless not getting enough water. If you fit a rope cutter the gap has to be around 40mm on a 25mm shaft - well above the 1 x shaft diameter "rule" and causes no problems.

Is your question because you have a large or small amount showing?