[glassdoctor]

Like Brian said the calc program is simple math. You can do the same thing with pencil and paper.

With a known kv of the motor and the voltage of the battery, either measured or estimated realisticly... you can get very close to actual speeds achieved.

The only thing you can't easily account for is the losses in efficiency of the esc and motor. If you assume a 20% total loss that's probably close in most cases.

I'm guessing most of the time that the calculator is off it is due to guessing the voltage drop wrong, or the kv of the motor isn't exactly what it's rated. For example Castles motors are not exaclty as they are labled... 4600 is not 4600kv but it's close to that. And the voltage while at top speed can be tricky to guess...