I bought that marking ruler to mark each side of the sew line and found that it created an oversized mark. The ruler did not work for me at all. In the end I marked the center line and used the 1/4" foot to sew along that marked line. That worked but I had to trim all of my HSTs - a lot of extra cutting.
Recently I bought the easy angle (and companion angle) ruler and it works like a charm. NO MORE TRIMMING. I love it.