It would be sooooo much simpler if you would just put a triangle at the beginning end of the binding. Start sewing about 6 to 8 inches from that triangle. When you get to the end, overlap with the beginning and you cut off that end at the base of that triangle, which is the width of the binding, no matter what width your binding is. No rulers needed. The measurement is right there in the triangle.
No need for miles of binding left over at the beginning and at the end. You need enough binding to match the perimeter of the quilt plus the length of that triangle. With most of us, that is 2 or 2 1/2 inches.