I have found some patterns have you buy too much fabric and others you have just enough. I have done patterns where if you make a little mistake there is barely enough to fi it. I usually buy what they tell me to buy, just to be safe. I don't purchase my backing fabrics until I am ready for it. Then I use my scraps from the quilt up making a back. Sometimes There are almost enough leftovers from the quilt front. I am trying to use up some of my stash this year so I often dig in to it. I have been making some really pretty backs. Sometimes my back is just a giant version of one of the blocks from the front.