Originally Posted by
GemState
It isn't the measurement of the seam that is the important thing, it is the measurement of the pieces you are sewing together. You want to press your seam then measure the sewn piece minus 1/2 inch (which is 1/4 inch from each piece) That should be the measurement that you are concerned with. Depending on your fabric, thread (and location of the moon and stars!!) your sewn seam can vary.
IMHO this is by far best method no-fail - thread, fabric can vary and the little flange on your 1/4" foot doesn't know the difference. I was taught to do three strips same width in different colors, sew and measure, then adjust, and it works great. I do it whenever I start a new project, change thread or fabric weight.