Jan, See post #6.
The calculator is for engine testing and comparing static load.
Tests have been done to evaluate plane speed, and almost always showed the plane to fly at pitch speed, so the calculator can also be used to predict plane speed.
Now, a racing plane is outside of the normal window, but still you should be able to get guidelines for the direction you have to move when carving that perfect prop that lurks inside the not so perfect one that is in your hands.
Of course flight is the ultimate proof.
But suppose you want to rework or blueprint your engine. Then you need to know where your gain was, and where your loss.
props are perfect air brakes for the engine, so you do not need an expensive engine test stand with torque measurements, as long as you keep a set of props for reference during the time of your engine rework.

Then you go up in the air with your improved engine, and match the prop to engine and plane for best speed.