Shamata - attention on upper lip, diaphragm or belly?

A very interesting discussion here on SE on breathing focus made me a bit confused. Allan Wallace says in the beginning focus on belly, then when you get better at it, diaphragm, then nostril/upper lip. He is basing his instructions (in *The Attention Revolution*) Kamalashila's *Stages of Meditation*. In the q/a i linked above some disagreement occurs there and some of the people there says diapragm is better. They are folks whom I've gotten very many good answers from before, and whse answers I therefore have come to trust quite much. But in almost al other books, videos, techers instruction the tendency is to say that once you get the hang of shamata, you should focus on upper lip/nose/nostril. If anyone has good references and arguments for focusing on diaphragm I'd like to hear them, and try it out.