I am with the little over 4 yards group. As you want to pull it over like a pillowcase, you need a double thickness for both sides. I don't know what flannel you want to buy but depending on the quality, it will shrink quite a bit so I would get at least 4 1/4 yards. And I would wash it a couple of times in hot water and dry on hot too so it shrinks up assuming you may want to wash it in the future. You will have extra off the width so you can use that to piece it if you want to.