I have also learned the hard way to buy extra fabric. I made a rail fence quilt and was instructed to sew four strips, width of fabric, and I would be able to get five blocks out of it. Now this was an Eleanor Burns pattern. I found out the hard way that fabric can be anywhere from 40" to 45" in width and I was only able to get four blocks not five for each strip set. When I went back to get more fabric they were out of some of it. I was lucky to find one fabric with enough yardage to make a wide border to make up the difference.
I wonder how companies can get away with advertising 44 - 45" width when it's closer to 42" or less.