Your formula may be correct, but is good only for comparision among propellers, instead of for the load a prop will put on an engine.

The increment in diameter has a bigger effect on the load than the pitch.
The load increases with the cube of the diameter (D X D X D), and the square of the pitch (P X P).

http://www.hooked-on-rc-airplanes.co...ropellers.html

http://www.ampaviators.com/2007/05/15/propeller-basics/

http://en.wikipedia.org/wiki/Propeller

(Editted to add links)