Here's a link to an electric motor calculator that might be helpful to some.
brantuas.com/ezcalc/dma1.asp
Being free, it is a bit dated and there aren't many modern motors, batteries or controllers to pick from...but you can enter your own custom values. I used it to get reasonably accurate predictions for the motor Chris has in his Wot4. Chris then used it too and was able to pick a better prop.
Mike