I generally expect to have 38-40" usable fabric after washing. I hope your new purchase doesn't shrink too much. If it does, perhaps you could just sew a narrow strip of a contrasting fabric, maybe something left over from the front, into the backing, making it that much bigger. I know it's a pain to have to piece your back when you had planned the quilt to avoid that. Gotta tell you, life's tough after 65! Have a happy birthday.