Originally Posted by Iceblossom

First off, if you have a regular quilter, ask what they prefer. Some don't care so much about backing seams -- others have some very firm ideas.

You are correct in your math that you want 60x70" for the back.

You might want to consider the "Flynn Method", I was looking for a handy visual explanation that I know I've seen before, but I'm coming up with John's and am concerned that the math might be intimidating. There are videos but I don't know what to recommend,
https://www.flynnquilt.com/free-lessons.htm

Basically when you are a couple inches off, it can be a better use of the fabric to cut it on a diagonal and slide up the pointy edge which makes the whole piece wider (and shorter).