My understanding 12" finished is when sewn into a quilt or whatever so when you finish constructing 1block prior to sewing anyrhing to it it should measure 12 1/2". I have found on quilter's cache that her patterns end up smaller so I move my needle to the right one click if you have a machine you can do that. If not check your seam allwance to make sure you are sewing a scant 1/4". This is what I have found in the past. Hope I helped
