You don't need any tool. Put a triangle into the beginning of the binding strip and put that where you want your binding to end. Start sewing about 6 to 8 inches away from that point. When you get to the other end, stop about 12 to 16 inches from where you started sewing the binding on. Take it out of the machine. Lay the binding where it will be sewn, with a little tug to keep it snug. Cut off ending of binding strip at the base of the triangle. No rulers or tools needed. Then pick up both ends and place right sides together in the same manner that you have sewn all other seams in the binding. Test this to make sure you have it right, then trip out the seam at 1/4 inch and press open.