I have always been concerned about running out of backing while quilting, especially someone elses quilt. Well, I experienced that tonight. You can see in photo that the batting was about 8 inches to small also, backing was about 2 1/2 to 3 inches but after it was finished it was only half that amount.
Next I got the iron and pressed the seam. I didn't want it to get twisted and make a wrinkle on the back of the quilt. After all that I rolled it back up and pinned the backing to the leader cloth. I didn't bother with pinning the top. I just smoothed it out and was careful when quilting it. I also laid batting under it and all this worked out just fine. It wasn't as much trouble as I had always thought it would be, just very time consuming.