I use the Quilt Calulator; I round up and add 10 total to the measurement of my quilts. exam: 53 x 72, I use 63 x 82. That gives me more than enough to piece my backing as needed, and as mentioned in previous post, all the extra goes into my stash:)
Good way to increase your stash when starting out. I also round up requirements posted in patterns. 1/2 yd, I buy 1 full yard. You know, if I make a cutting error I have it covered.