Welcome from Ontario, Canada. I don't do math because I never trust my calculations. If I have a large quilt that I don't want to piece, I use the extra wide backing fabric. If I do piece the back, I usually put the seam vertically because it is less noticeable (in my opinion) if you get it a little off when making the sandwich. It does help to put pins on the centers of both top and back so you can feel through the sandwich to match them up. I do like the backs where they piece in extra blocks to make the quilt reversible. The only drawback I can see to that method is the quilting from the top will not match the design on the back?