I am so glad to have read this. I'm working on my fourth quilt, all baby quilts. The directions said to take the fabric and fold it lengthwise and cut it crossways, or something like that. I cut the material the way I understood it and sewed the two pieces together for the backing. When I laid it out, it was too short. I was so frustrated because I couldn't figure out what I had done. Now I'll have to piece the backing together.
I'll use these suggestions in the future. I thought it was just my math "inability" and it makes me feel good that other people have problems too.