The center is at 1 1/8" use the side of the ruler without the 1/2" and just go in 1 1/8" from both sides and that is your center. Cut the two sides, rotate and the line up and cut the other two sides.
Also make sure your diagonal seams line up at the corners, which are going to be 2 1/4" up each side. Sometimes it's better to have the center a hair off than the corners.
I prefer to use larger rulers when square blocks, and it was fun with the 365 block challenge as I had a wide variety of measures that needed to be square up, sometimes as small a 3/4" HST or 1" x 2" flying geese. I had a lot of weird sizes because they used 365 different blocks, and all had to fit into 3" or 6" finished. There were a lot of 4 base blocks in the mix.