If I'm not sure about the fabric, I buy 1/4 yard, and take it home to see if it "works." I can always use the 1/4 yard for church projects.
On my last quilt, it needed queen size backing plus the 4" for the LAer. SO, I went to Walmart, bought a cotton sheet for $11. Took it home, washed it and the LAer was very pleased with the sheet fabric. It was soft and worked up well.