Make your four patch borders and measure them before you try and figure out the plain (coping) border. after you have the four borders (2 sides, top and bottom) measure them and you will have some math to figure the coping border. there are tutorials on line
http://quiltyfolk.blogspot.com/2014/...-when-you.html or come back and pose the question and someone will do the math. It's a beautiful quilt.