I think that ruler/tool is one of those that you definitely do NOT need. Just draw one diagonal line down the center and sew 1/4" (using presser foot as guide) on each side and you're done. Why waste time drawing two lines.
And always cut your squares 1" larger than the finished HST needs to be. I've found that no matter what tool I use or how careful I am if I cut the squares at that 3 7/8" (or whatever 7/8" they tell you) at least one of them or both of them never come out the right measurement.