By binding ruler, do you mean to cut the binding itself or to join the binding when you're putting it on the quilt? I don't have a ruler for either one. For joining, I just trim the overlap to the width of the binding minus 1/8" (before I did that it would be a bit loose, but I use bias binding so it may vary with WOF binding) and sew it on the diagonal. Like this:
https://completelyblocked.wordpress....-binding-ends/