I can see that there is no cut and dry method of positoning the home button assembly. The screen I am replacing was shattered around the home button, so when I was putting it back together, I had no reference for placement. So....I got it too far towards the edge of the frame, then it wouldn't fit in the opening, when trying to place the screen on the frame.
I am in the process of replacing a shattered screen on an iPad 3 wifi, and I literally was removing glass piece by piece! I do recommend having a roll of wide clear sealing tape at hand. Then, when you see loose glass shards, pull off about 4 to 5 inches, and stick the tape to pick up the loose glass, and once in a while, fold the tape onto itself (sticky to sticky side), and set it aside. This helps keep it from getting everywhere. Also, spread news paper under your work. Some people don't really think of these things, but the fact is, hardened glass will find its way into your skin very easily. Extremely sharp!