Also, when I square up my blocks, when squaring up separate pieces like flying geese or hst, I will use the bloc loc ruler for it and when squaring up the finished block, I use a square ruler the same size I need for the finished block. I also use the rotary cutting mats. I have a 14", a 17" and a 24" rotary mat. For the finished square rulers I have the 1" all the way up to 20.5 inches, with all the 1/2 sizes in between. I bought them as I needed them so the cost was a little at a time.
I just lay the square, with the center mark on the square to the center of my block and cut the right side and top, then rotate the rotary mat without moving the ruler or block and cut the other two sides. As long as I am careful of match the center of the square with the center of the block, my blocks all come out in the end the exact same size.
Added to note: I do QAYG and I DO NOT square up my finished block until it is quilted. So when I am squaring it up, I have three layers. The top, batting, and backing.
When I prepare my blocks for quilting, I cut the backing 2" larger than needed, the batting 1" larger than needed and then place the top. So when I quilt it, if the batting or backing pull in slightly because of the amount of quilting, it will still be larger than the top. Then when I do the final squaring up of the block, the entire sandwich is squared up at the same time. Then I just sash the blocks together, then the rows together and put on the binding.