Originally Posted by wolfkitty

Fold opposite corners together to make a triangle - that will tell you where you are off. If it's square, all the sides will match up. Fold again to give yourself a smaller block to work with. I think you would have cutting edges together, but it's late and I'm not sure. (3 a.m. here)
Anyone, please jump in here if I am steering her wrong.

Originally Posted by smitty

you will probably need to take a little off all the edges. you need to find a way to press or chalk mark the diagonal. then use that line, not the edge of the block. or you can use a straight seam line & measure from that all directions. man, the more I type, the more complicated I am making it. start with the smallest block & make them all that size, unless you will lose points. you may need to make another block or two to get enough sized ones.
guess I would measure 10 1/4" from the center to each edge. you would be working in quandrants. that way, you could use a smaller (12 1/2 ") square ruler. this is probably the biggest block you will ever need to square !