I also have the 6600, and like seamstome, would invest in the 1/4" AccuFeed foot.
If you don't want to tape on your machine, I would use your current AccuFeed foot (I have an open toe one that I'm currently using, to get my hands and brain use to making an accurate seam), and practice with some scraps making a 1/4" seam. See where the 1/4" may line up on the foot, and mark the foot with a permanent marker.
There is also on the quilting market, an adhesive stick on ruler marking that has different measurements, and sticks right on or near the bobbin plate of your machine.