first, the 12.5 by 17.5 size is not very big for a placemat - so, as Sadiemae asked, what size do you want them to be.
Next - if you do want it smaller, you could avoid math calculations by drawing it out on graph paper and then just measuring the pieces, add seam allowances and cut the fabric out. (Do this on graph paper that is 4 squares per inch to make is easy.) To make it square, you could just use the center nine-patch block and eliminate the coordinating block on the sides.