I use Frixon pens to mark my line. I make my initial squares at least 1" larger than my finished square so I trim the edges after I sew on either side of the line and cut on the diagonal line. Since I am using the sewing line as my guide when trimming it doesn't make a lot of difference if my sewing line is exactly 1/4" away from the drawn line since no one will notice a slight variation in the seam allowance which is hidden on the underside. A slight variance in that will not affect the size of the finished HST if you use the seam line as your guide for trimming. I have Quilt in a Day HST ruler which you use to trim before you press. I really like that ruler but any ruler with a 45 degree line on it will work for trimming if you press open first.