The
tiler starts with planing the layout of the tiles on the wall, marks out where he wants the alcove, and then installs it
Don't worry, it's a common misstake. What you need to think about now it to use the alcove as a starting point. You need to "frame" the mosaics on the edges of the alcove in a way which goes well with the rest of the wall, but as the alcove is the centerpiece, making a symetric "frame" is top priority.
If you can get whole tiles on all edges, or a half tiles on all sides with L cuts in the corners, you've done real well.
If that's not possible, but if you can get, say, whole tiles on the upper and lower edges, and 2/3 of a tile on both the left and right edges, it's ok, as long as it works with the rest of the wall. 1/2 a tile is minimum when dealing with mosaics in my opinion.
It's a bit hard to explain without pictures, or actually being there, but I think you get the gist.