I was told once that the purpose of finish carpentry is not to be perfect but rather to make it appear to be perfect. I doubt that anything is truly square, plumb or level in a house, so you fudge things a little here a little there so it looks like it is. I had a 4' x 4' shower that was 1" longer at one end than the other. I used, 1" square tiles and each 2 or 3 rows, I added a 1/16" to the grout line. I have a porch that is 2" wider at one end than the other, every couple of rows I add a little to one end. Chalk lines are very helpful. Measure many times during the construction so you can adjust as you go.