I have the 1/4 inch foot and love it. I checked it on the index card and it is right on. So to get a scant 1/4 inch I just move my needle a hair (know not everyone can do that)
to the right and check it again and have my scant 1/4 inch.
If it is something I have to draw a 1/4 inch seam line on then I sew to the outside of the drawn mark and it usually is a scant 1/4 inch. You have to kind of play around with it if you can't move your needle.
I have one machine I can't move my needle except for 3 positions, so I find where the scant 1/4 is and then take either masking tape or blue painter's tape and tape down on my machine bed as a guide, works great.
The scant 1/4 inch seam is just a thread shy of the 1/4 inch I have been told and have found it is pretty close. Be sure to do a good job of pressing, that also helps. I have seen people press and the seam not be pressed completely to the side, so check your pressing closely it helps greatly. My two cents on the subject for the day.