Warren - if you wanna be so darn certain then you need to get some weighing scales and attach it to the plane and give her all she has got. Another way is to use ThrustHP as a mere guide and discount say perhaps 15%. I use that as a guide and it seems to be about there when I remove 15%. What Bodywerks has done for you is pretty accurate (its through experience) if you were to use the most preferred prop on the motors he has stated. Most preferred prop can be had by going to the Tach forum or perhaps asking around here.