The problem is everyone talks about a 1/4" seam, or a scant 1/4" seam, however, you really don't measure the SEAM you measure the resultant PATCH. Here's a way to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
You figure out where on your machine, with the fabric & thread you are using results in the correct patchwork and that's where you sew. The other thing to keep in mind is if you change fabric, go from cotton to homespun for example, or change thread weight, you should recheck your seam because you may have to adjust it.