when i use the regular 1/4" foot, the machine default (3.5) gives a perfect 1/4".
for scant, i go to 3.7 or 3.9 depending on how thick the fabric is.
i piece with 50wt thread in top and bobbin. if you piece with 60wt thread you might be able to use 3.7 or 3.8.
my favorite size needle for piecing is 10/70, although i have to switch to the 80/12 for batiks.