I measured with a very precise Incra rule (
http://www.incrementaltools.com/category_s/157.htm ). With size 50 thread and cotton quilting fabric, the thread takes up about 1/32-inch. So I get a perfect piece if I sew at 7/32, or just past the right edge of a pencil marked seam line. I fiddled with my needle and foot until I had the settings right and then marked the front of the foot. This is my New Home (Janome) F foot.
My ancient LittleFoot was letting the fabric slip all over the place.