I put an omnigrid ruler under the pressure foot and SLOWLY lower the needle just to touch one of the lines. Then I mark 1/4" away with a sharpie. If you want to really be precise, you have to take into account the width of the thread. If you use Connecting threads thread, you will have to have the sew, be a bit larger than if you use John Flynn's So Fine. When you fold the seam over to press it, the bigger thread takes up more space than the thinner thread. Again, though, it depends on precise you need/want/care to be.