Originally Posted by PaperPrincess

It's common to over cut half square triangles, then trim down, but this is probably a bit more than you need. If you haven't cut all the squares yet, I would do a couple of trials, one using an 8" square and one using a 7 1/2" and see which one you are most comfortable with.
You normally add about an inch to the finished size, which allows you to cut a sliver off to square it up. If you are supposed to trim to 7", the finished size is 6.5, so a 7.5" square would be a more common measurement to use.