Thanks, all, for your advice. After moving the needle over more and pressing more carefully, I was able to get a 12.5" width, but then the other side of the square was still 12 3/8" (which had to do with my cutting, I guess). Aargh! It was time for a glass of wine at that point. Lol. But then I came back and read your note Jan, and I decided I'll just square everything up to 12 3/8" and get on with it.
By the way, thanks for the blocking technique, Feline Fanatic. I'm sure that'll come in handy sometime.