When teaching new students about setting up their sewing machines, I use this technique:
Cut 3 pieces of fabric each 1 1/2" wide by 6" long with 1 piece from a different fabric. Sew the 3 pieces together with the contrasting fabric in the middle. Press seams and measure the width of the center piece. If it is 1", you have your perfect 1/4" seam. If not, you need to adjust your needle position. I have them use a longer stitch than usual so it is easier to rip out and sew again. Keep trying until you get it, then write all your settings down for the next time.
I have (and love) the 1/4" foot with the little black "wall" against which you run the fabric to sew. HOWEVER, the needle position still needs to be set. For my machine, I need to move the needle from the default setting of 3.5 over to 4.2. On my older Janome, I don't need to move anything - go figure!
A scant 1/4" is only the difference of about a single thread's width. This is done because when we press seam allowances, the fabric goes up and over a teeny, tiny bit before lying flat, so we need to allow for it's detour by taking a teeny, tiny bit smaller seam width.