There is a marvelous 1/4 inch foot that is clear plastic and is also marked with 1/4 inch horizontally at the needle position as well as 1/4 inch behind the needle and 1/4 inch in front of it. (Is that clear as mud? )
It's made by Littlefoot and available at some LQS as well as at
http://littlefoot.com. I got one this summer and LOVE it.