Just throwing it out here but, if the difference is the size of one thread, it should not be making that much difference in the outcome. Especially if all the blocks are coming out one thread smaller. If all the blocks are coming out the same size the pattern should work just one thread smaller. Does that even sound English?? In my head it made sense!! LOL. If you have a quilt, that is 10 blocks across, each block is one thread off (it's a given that different thread will be different thickness) that would equal 10 threads, approximately a quarter inch or so. Over the whole width of the quilt, 1/4" should not make that much difference.
Sometimes, we are sooooo concentrated on that one tree, we miss the whole beauty of the forest.