hey paul, there really is no magic number, it is always variable if the plane is moving toward or away from you. it depends how close you are. Sometimes I tend to not go in tight enough because I fear "losing" the plane but I really should zoom in more. it is tough when the plane is moving. I would say you'd have to be wide when a 74 is coming. I have shot 747s side shots at 56mm but that was straight on the side.
Best advice is get in as tight as possible. But then, if you fill the whole frame and then find later you need to slightly rotate it to level the horizon you're screwed because you'll end up cutting off a piece of the horiz. stabilizer or wing tip.
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens