Originally Posted by Bobbinalong

For a diagonal join you will lose a little more than the width of your binding strip.

Your diagonal stitch line goes from corner to corner of an imaginary 'square' IYSWIM, and when you open it out you have 'lost' the length of the overlap. The extra length of binding that you add to your calculation is one 'overlap'. Overlapping the final ends of binding and cutting it longer by the measurement of the width (eg an extra 2 1/2") is how I make the final join of the binding when attaching it to the quilt. Easier to do than explain. You will also lose a little binding with the mitres on the corners.

Left over binding always come in useful for scrappy binding on Linus quilts or for scrappy patterns.