You should engage your pressure foot and insert the needle on the line to find your quarter inch. From there you can butt up a stack of post - it notes (some use other materials such as tape) along the straight edge of the index card which acts as a seam guide. Place this just in front of the feed dogs. Using the post-it notes as a guide, sew a seam on a scrap fabric square and then measure to make sure it is 1/4 " throughout the seam. Most define "scant" to mean a threads width from true quarter inch to allow for turn of fabric, so you might have to adjust several times. Clear as mud, right? :D I hope this helps. You can also use a smaller quilting ruler in place of the index cards to accomplish the same thing. Also might want to consider purchasing a 1/4" foot for your machine. Let us know if you found your scant :D
Cindy