Am using a master airscrew 16x6 on a ys.91fz and turning 8700 to 9000rpms. Thrust cacluator says 16lbs of thrust. Using this combo on a h9 p-51. Pulls like a 747.

Well there you go... and remember a Master Airscrew is a pretty grim propeller.

You might want to try a Graupner prop. It will give you better performance.

Try 0.6 for the CF on the calculator. 1.0 CF is assuming 100% efficiency. APC gets like 70%, and MA is like 60%. So pick "custom" prop, and put 0.6 or 0.7 for the CF. That will be more like the real result. I correlated with other people's thrust scale reading to get this approximation.

For the YS 91, the mfg recommends keeping the peak rpm at abotu 10k, so you'd achieve this with a 15x8 APC.