Oh, Sir, I am not laughing at you; Please excuse me, that was not my intent.
The laughing is at the Customs people telling you that some flight time conversion factor is in the "FAR/AIM". I thought someone was yankin' your chain, but then realized it may have been a genuine reference to the FAR regulating time.
The military interpretation of "moving under its own power for the purpose of flight" is the same as the old civilian way, before the introduction of the hour meter installed in civilian rental airplanes.
This meter starts at engine start and runs til engine shut-down. Since civilians are charged flight time rates for all that time, that's how they have justified the interpretation to mean from "beginning the taxi", which still doesn't include the dead time from engine start to taxi, but they log all of the "meter" time, so some companies will factor in a little extra for standardized comparison of flight times, but there is no government publication of this conversion factor. That I know of.