I may not have answered the first question completely.  You can't use any of the maximum wattage or current figures provided by the motor manufacturers to calculate anything meaningful because there is no propeller involved in those numbers.  The propeller affects everything in the drive train just like it affects the rpms of your fuel engine.  That's why I sent you to the Scorpion web site to look at the performance charts.  They will show you how watts and amps vary with the propeller and battery connected to the motor.  To calculate what you want to calculate you need the actual measured numbers with the motor spinning a prop, not some maximum figure provided by the manufacturer.