I was told of a much simpler formula:

Double the length and add the pitch.

10x6= 26

I have applied this when trying to find the best prop for several different four strokes, from .52 to 1.20 size. There is some validity to it, because everytime I got too far beyond the "median" number, in either direction, the engine either bogged or overspeeded.

As an example, the "median" number for a YS 63 is 32, since a 13x6 is about the best general-purpose prop for that engine. It was "happy" with everything from a 12x8 to a 14x4. It did NOT like a 14x6, and wasn't "happy" with a 14x5.

So, yes, I think this is a useful thumbnail comparison factor.

Steve