ORIGINAL: MikeL
That's bad advice. It assumes that the power draw on the ground is the same as that in the air, which it most certainly is not. On the ground you're not putting the same physical load on the servos. You're not accounting for vibration nor the power required to move and hold the surfaces when there is considerable airflow over them. The three methods listed above are a fine way to calculated the power drawn in each of those three scenarios, but a very poor way to measure what is actually drawn when flying the model. Following the above advice could very easily result in the loss of a model.
Bad advice?
Hardly, though I know where you are coming from and do not disagree in that you must be careful.
There was a great article about servos in FlyRC last month which talks about servo draw, etc. The variance between running the servos statically but mounted on the plane with linkages, etc., versus in the air was not that great enough to have a highly significant impact.
Maybe it is not accurate at worst, but like glow fuel run time determinations you ALWAYS allow for error.
e.g. if your ground time is say 2 hours, then it would be quite foolish to fly for 2 hours w/o checking.
In this scenario I would take say 60% of the measured time on the ground as a max and work from there.
Personally I prefer charging the batteries and measuring the amount of charge they took after a day at the field.
Needless to say I never bother with the 600mAh or less TX packs, and I always tend to use at LEAST 1200mAh packs or larger on .40 planes and up.