You don't have to use a ruler to measure how long the overlap should be. Your measurement is right there on your binding, no matter how wide the binding is. Just fold a triangle into the beginning of the binding, and the base of that triangle will be the width of the overlap. Cut the tail off at that base and you have the correct length.
Also she doesn't show the seam after it is sewn and pressed open. There should be a square on the back of right side of fabric. Then you trim that seam, refold the binding and sew it on with the seam open.