:) I hope it's helpful to know that Bernina has 2 quarter inch feet. The first is a #37 and the other is a #57 that has a guide. I have both and use the #57 all the time. It gives that "scant" 1/4". Before that I measured w/a ruler under my needle and stacked up masking tape to guide the fabric. Good luck!
PS: I see you're looking for a 1/4" Walking Foot. I guess you could use the masking tape or measure the Walking Foot, there may be a mark on it you could use. I seems I've done that before.