Here are a couple of shots of my ironing and cutting table full of scraps and piles of fabric. I find that I can think though the process better when I see all the fabric options that I can use.
The tissue paper on top of the pile of fabric is a quick and dirty 12.5" x 12.5" pattern. I used it to test if my block was big enough yet to square off. It was easier to guage which sides I needed to add more rows to when increasing the size of the block to match the final deminsions. It worked out well for me. I hate to have to recount the inches on my cutting board to tell how big something is. I often make mistakes that way by miscounting. Not good ;) I don't have one of those big square cutting guides, so I think this will do for now.