Well it depends on the teacher who is guiding you. If you do have a teacher go by what he instructs.
One of interpretations focul point (as in the original teaching as well as the most effective location) is the centre of the upper lip: What is the Interpretation of Parimukham in the context of Buddhist Meditation?, Is watching the heart beat (instead of watching the breath) a known form of meditation?,
Also a very comprehensive book which gives instruction to the original breath meditation method see by answer to: Breath getting shorter and shorter and also Is watching the heart beat (instead of watching the breath) a known form of meditation?
When you focus on the diagram you are supposed to look at the Wind Element, i.e., the expansion and contraction. For a beginner this is generally easy but there are arguments that as you go deeper into meditation you progress stall and slows down for some. Based on this rationale have tend to switch to other methods which seemed to have worked better for them. (E.g. Banthe Vimalasiri, Anagarika Munindra, Piya Tan, etc.) This might not be the case for everyone but you will not know for sure whether this will work for you or not unless you try it and see for yourself.
If anyone has good references and arguments for focusing on diaphragm I'd like to hear them, and try it out.
Also when you have developed some level of concentration and also looking to switch to 4 elements meditation while doing normal day to day work this might be sometimes a good place to rest your attention. (E.g. in office, while driving, etc.) For more details on which parts of the body which each element dominates see: Mahā Rāhulovāda Sutta
For a basic introduction to the Mahaisi Method see: Essentials of Insight Meditation Practice by Ven. Sujiva. This was a book I used for reference when I tried out this method many years ago. Also there is a book: How To Meditate by Ven. Yuttadhammo.
Meditation techniques are like drug trial. There are some methods with generally work well for everyone but with a few exceptions. There are some methods which works well for a few does not give as great results to every one in general. Keeping this in mind try to figure out what works well for you. My belief and experience is that what works better to the masses is when you practice close to the original Suttas. A view shared by: Banthe Vimalasiri, S.N.Goenka, Ven. Nanavira, Richard Shankman, Leigh Brasington.
Also keep in mind you have to pratice for a while using one techniques to get results. You should not change too often. Only after a year to two with no progress.
Best of luck in your search and pratice!