If you are having trouble lining up you should measure until you can accurately "eyeball" it. Get yourself and old fasioned seam guage to measure. It makes quick easy work of it as you set the guage to 1/4" and all you have to do is hold the guage area to the "ears" to do a quick and easy measure. You can pick them up just about anywhere for a couple of bucks.
It shouldn't take you too long to eyeball it accurately. It is a valuable skill to have as it comes in handy when peicing diamonds as well.