There are a number of very good computer programs for calculating electric motor or electric model performance, based on the motor mfg, motor model, prop size, number of battery cells and so on.
There are a lot of other similar programs out there, both free, and not free.
So, could the wattflyer readers post the program(s) they use, and its corresponding web site? Perhaps, also include a list of the good things and bad things about the program you use. This would be very useful for all of us.