The thickness worked out fine at the corner with the other wall, but the patch is at least an 1/8" too thick at the other side. Plaster, since it's hand-applied instead of factory-made, can vary in thickness unless the plasterer is very careful about even thickness; I can only assume that's what happened here.
Since the whole thing is getting covered with a tub surround, I don't think it's going to be too important. My tub surround might be a little lumpy, but it probably won't be very noticeable. Another thing to note, ex post facto: mark the location of studs before putting your wallboard in place, and you will curse less and waste fewer screws.
Also, turns out my rock-solid faucet, while still rock-solid, is not as straight as I had thought. I should have been more careful when fastening the straps. The cold water tap projects from the wall about a quarter-inch more than the hot. That's invisible when there's no wall, but now it's clear. Sigh.
Well, if that's the worst screwup I make in this renovation, I'm probably going to be pretty happy. At least it all works, and doesn't leak.