If I do a ride at 15mph, then ride the same distance at 19mph, the kcal/mile shouldn't be the same at all. The difference in time for those two rides grows linearly, but the effort grows quadratically thanks to wind resistance. I'm unconvinced that the formulas using only HR factor this in properly. As others have already said a bunch of times: use the figures from the power meter. It's the only thing you've got that's actually measuring (not guesstimating) the actual amount of energy that was used to push those pedals. I just got a power meter last week, and one of things about that that pleases me the most is that I finally have some calorie burn figures I can trust. I'm using Garmin head units, and in my rides so far with the PM they seem to be using the kjoule figures appropriately.