What works for someone does not always work for others I've found out. Some cottons are thinner than others so when you fold a seam to one side, it takes up less room than heavier cottons. My machine's foot has a little looseness so that can make a seam off too. While any little thing doesn't matter much, a lot of little things can really get your measurements off. I think you have to figure out what it takes to stitch a 1/4" seam on your machine, using your rulers, and your markers.