First, Jan in VA, I always appreciate your pictures. I suppose I'm more visual and you've helped me understand things several times.
Second, I was just struggling with figuring out the math for a backing earlier this evening so I sympathize with you. I have no advice or input (if I did, I wouldn't struggle so much myself), but wanted you to know you aren't the only one. Why is it that I can make an intricate top with a gazillion pieces, but can't make a backing with 2 or 3 pieces?