I added one-inch extensions to each side seam on the opened side. I figured that way, not only would that give me more fabric to work with on the closure, but if I messed up, those small pieces would be easy to replace.
For ease of dressing, I opened the side seam an inch past the underarm seam. I sew in my sleeve caps first, then sew the side seams, so theoretically, I could have the side open from hem to cuff. I don't know how having a button under the arm would work, but with the snaps, it doesn't seem to be a problem, although I only finished this last night, so who knows what will happen during a day's wear.
The body of this pattern is in my perfect spot of fitted, but still loose. Even though I like the look of this side closure, it's a little too inconvenient to get the shirt off for me to prefer that method. I need to learn to make a classy front-opening closure without button-holes.