I can get a pretty consistent scant 1/4" using a lined 3x5 note card. The lines are 1/4" apart -- if you put the needle inside one of the lines and line up the line with the edge of the foot, you get a scant 1/4" for some fabric. I more frequently line up the needle on the inside of one line and the edge of the foot on the inside of the line immediately to the right. I like this better than using a plastic ruler as I don't accidently "ouch" my needle or ruler.
As always, take the time to make a test with your fabric to see what works the best.