Hello if you want your quilt to fall apart 1/8" seams will do it! You can fudge a tread or two but never 1/8" 12.5" means 12.5" easy to check your self and make sure that you are sewing that 1/4" seam. Also accuracy means having balance in the thread you use. Do not use a heavy thread piecing it will automaticly throw off your seams. Sorry machinists or engineers we are not but accuracy counts for everything. We can always go bigger and trim to exact but smaller is just that smaller it dose not get bigger no matter how hard we try. I have received a lot of blocks from swaps or raffles, and there are always the ones that are small don't ask me why that is the challenge to be able to use those blocks. I love the wonky log cabin to be able to enlarge the blocks but wind up with a nice quilt.