You can do this without any tool. Fold the beginning end of the binding down to form a triangle with all raw edges at the quilt edge. Press very well. Fold binding in half as usual. Leave a 6 to 8 inch tail at the start and sew on the binding. When you have 12 to 16 inches left to sew, take the quilt out of the machine. Overlap the two ends of the binding and cut the end off at the base of the triangle. They will overlap just as much as the binding is wide. Then you can lift both ends, put them right sides together, and sew them as you sewed every other seam in the binding. The pressed line is your sewing line.
This method will work no matter how wide your binding is. The triangle is always the correct measurement.