perhaps part of your difficulty is looking at the unfinished size of the center. It will finish at 64"
11 of your 6" (finished squares) will be 66" The difference is 2" finished inner border. So the posts above are correct in suggesting 2 1/2 inch cut strip for your inner border. Be sure you measure the center down the center to make sure it is square so the inner border will keep it in the square.