OK, I tried the math approach, but it gave me wonky numbers, so I went upstairs to check out the pattern. (It's one that's on my to-do list, so I was self-motivated :mrgreen: )
I found that the fourth square was off by 1/16" It calls for a 4 1/16" but the 4 1/8" seemed to work better. I also made the last square bigger so I could square it off.
Using a 1/4" seam allowance, my final square was 9 1/2"
Are you using a 1/4" foot?