I never said that the results were inaccurate. They may or may not be, depending mainly on how well the assumptions that the program makes match your model. If it is a very unusual design, the results are likely to be less accurate. But they are almost certainly more accurate than if you just guessed.

There are two ways to effectively use the calculator. One method is to use the results to compare the relative merits of two components. If propeller A looks like a better match for your model than propeller B, it probably is. The other method is to calibrate the results to exactly match your test data. The calculator is still very useful without this extra step, but it is always there as an option.