Hi Mrs Toasty Koasty,
As someone as mentioned refer to John Flynn's backing instructions. Using diagonal piecing for all or part of the back can save the amount of fabric needed.
There is a practice where there is a decorative section across the top of the back of the quilt. This decorative section can be filled with extra or practice blocks. This area could be filled with a piece of every fabric used on the front of the quilt. This area of extra fabric is then available to be used to mend or repair the front of the quilt if needed.
Another suggestion would be to include another section across the bottom of the quilt using left over fabrics used on the front.
Also be sure to make a large pillowcase in a size that will hold your folded quilt for out of season and regular storage.
Good luck with your backing.
Pam M