Exactly!
Not much better than a guess, but it can give you a bit of an idea of what type of performance you may get.
The calculator I used is really just a design tool it gives you a rough idea of what type of performance the prop may deliver. For example one thing it is good for is, it said the required HP needed to achieve those type of prop pitch speeds would be 1.2 and 1.4 Hp respectively to spin the prop up to those rpms (I would guess that is using the drag of a average type model for a reference). Now that is something that's worth knowing if you wanted an electric motor to spin those props, now you know it would take a 900 or 1050 watt power system to do that.
One of the other things you can glean from it as well is that (at least with a speed plane) if you can't get to at least 80% of that figure something is wrong.
