None of these prop load rules of thumb work very well. Dia x pitch probably works as well as any of these. The problem is that efficiency is not just Dependant on the prop design. For the same prop and engine efficiency is highest when the plane is sitting on the ground with the engine at idle. That is it moves the air at almost the same speed as the prop pitch. The efficiency is lowest when the plane is in a dive, the air is turning the prop and has a negative efficiency. When the plane is at full speed straight and level it would have a much lower efficiency than when running full speed on the bench. For the same design efficiency goes up with more diameter and down with more pitch.

One thing I see people do that is wrong is assume that RPM * pitch = plane speed. That is wrong. For one the prop pitch is not constant, it is the average pitch along the prop. The inside of the prop is at a much higher pitch. If efficiency is always 100% then the pitch * RPM would give the average speed of the air flow through the prop. If the plane is at full level speed and efficiency is 100% then the prop would have an average airflow through the prop that is RPM*Pitch+plane speed. That is the prop accelerates the air at that velocity, it is not the limit of the speed of the plane, it can be higher but usually lower.


Here is a prop tech site you might enjoy!

http://beadec1.ea.bs.dlr.de/Airfoils/propuls1.htm