To make sure I am getting a perfect 1/4" seam, I cut 3 pieces of fabric at 1 1/2" wide by whatever length - 6" or so. Set your machine for the 1/4" seam you usually use & stitch all three pieces together on the longer sides. Press the seam allowances all toward the same direction. Now take your ruler and measure the middle section. If it is exactly 1" side, you've got your perfect 1/4" seam. If it is too narrow, you need to move your needle to the right. If it is less than 1/4", move it to the left. Repeat sewing 3 1" wide srips together until you've got that 1" middle sction. Write down what your machine's settings for this are & put it where you can see it!
I have the Janome 6600P & you'd think that after spending $2000 for it, I would get a perfect 1/4" with their 1/4" foot. Well, of course not! The default setting for needle placement is 3.5. I have to move the needle to 4.1 for a normal 1/4" seam & 4.2 for a scant 1/4" seam. I love my Janome so I don't mind moving it round.