First of all; save yourself sometime and just do it at 7in and then square it up. But the 7/8the mark is the easiest; it's the last one before the inch mark unless you have 16th in marks on your ruller. Just remember it takes 8 8ths to make an inch so count over to 7 and you'll find that it's the last one before the inch. If you have the 16th marks they are normally smaller than the 8 th marks are. I went thru the same thing when I started sewing again.