For a perfect scant 1/4th inch seam, I use a product called Q-tool sewing edge. They are re-positionable vinyl stops
that you 'bump' your fabric up to when sewing your seam. On my machine, you have to remove it to change bobbins.
But, after placing full bobbin into machine, to match my already sewn seam allowance, I place a sewn seam under the
presser foot, align the needle with the seam, then align the q-tool to the outside of the sewn fabric. This product makes
it so easy to do the dreaded scant 1/4th seam....I got mine at my local quilt store but the the product card gives the
an email address of
www.aliciaattic.com. I'm sure if you Google Q-tools you'll find them. Makes my seam prefect all the time, and it saves me time (and worry) when it time to assemble the final quilt. They do lose the sticky that is on the back of them, but I just put double sided tape on them, works great. Used them for years and would not be quilting if I had not found this product. ( I do not work or I'm I affiliated with this product, just happy I found them)