That seems like an interestingly easy calculation. I would agree that the prop pitch and RPM would be able to get you a maximum theoretical speed, but I'm not sure that is all you need. The pitch is a measure of distance traveled during 1 rotation in an incompressible fluid, with 100% efficiency. Since our props usually move at upwards of 0.2mach (at least in places) you definitely have to deal with compressibility effects (Prandtl-Glauert equations). When you start dealing with compressibility, the air viscosity becomes exponentially important.

In addition to that, you have propeller ineffeciency (which is huge in air), wake rollup, tip loss etc that all must be factored in to the props' "maximum speed". I know that number is just an estimation, and without doing the massive calculations you have no actual way of knowing, but that estimation could be substantially off if it was calculated for full scale or even a slightly different scale. One thing that seems weird is when Irun the rcpro calculator for any of my typical set-ups, they give me a number that is roughly my actualized cruise speed. Theoretically, the number from any calculator should be the propeller's max speed if it were by itself...adding the drag of the aircraft should put the max drastically lower than that, and in the OP's case well below the stall speed Iwould assume, unless the calculator is attempting to account for drag count. Seems fishy.

To use the cliche bee metaphor, any calculations done at even our scale wouldn't allow a bumble bee to fly. Viscosity (Re) is everything, especially between 0.2 and 0.9mach.