Lines on a standard index card are exactly 1/4" apart and are straight. I cut the tiny space on the bottom edge off with paper scissors, turn the card upside down so I do not see the lines, practice sewing. Turn card over and if sewing on the line it was straight and 1/4". Have to cut the card down to the next line to practice again. Cards are so inexpensive. I do this with most every sewing session unless I know I have the machine set correctly for the 1/4".
Practice and watching quilting videos on you tube have helped me so very much.