That pesky 1/4" seam. You really don't want to measure the
seam, you want to measure the
patch that is the result of sewing and pressing the seams. Here's one of several great tutorials on this board that explains it better than I just did:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Having said that, for this project if all your blocks are consistently 12 3/8, and all the blocks in the quilt are square, then I would square them all up to that measurement and complete the quilt. You can sort out your seam width on the next one!