I agree with MTS, but if you're not in a rush you should try and figure out why it's not coming out 12 1/2".
Start with the cutting and make sure you are getting accurate strip widths. One famous quilter (I think it's Jinny Beyer) suggests always cutting one thread larger than the measurement.
Have you checked to see if your 1/4" seam is actually a true 1/4"? Your foot or needle may need to be adjusted to get the accuracy you need.
What size is your block coming out? Is it 12 1/2" in any direction?
Hope this helps!