My math also fails me often, especially when I'm in the fog. For me, the easiest way to make this larger would be to add a couple borders. Perhaps end up with 3 borders with the center one being narrower. It would also look good with this pattern, IMHO, to do the 3 strip border and use a 9P in each corner. In that case all 3 borders would be the same width as the squares in your 9P.