If you need more cloth across the fronts, letting out the center back won't contribute much. You didn't post a picture so the issue at the front button may be related to something else. Take it to a tailor and ask. Yes you will get 1/2' per seam. To get more cloth on the fronts you might need the sides let out and the front shoulder squared uo to pull the cloth forward where it is needed. Just guessing.
3/8/15 at 12:10pm