When I measure for the mitered corner - I lay the border strip wrong side down, move it towards the left side of the quilt and then fold the end up -so it makes a 45 degree angle. I usually give it an inch 'tail'. that always works for me, so I don't have many inches left over. I know some say to mark the strip in the middle, and place that in the middle of the quilt - that wastes a lot of border fabric IMHO. I have even used my method when doing two colors in a border strip. I use mitered corners on all of my projects now, unless I am doing a corner stone.