The easiest way to calculate the D9P is to add up the 3 unfinished size blocks and subtract 2" to get the finished size. I've got a couple right here that I'm working on and measured them.
4.5" unfinished x 3 = 13.5" (finishes to 11.5" )
5" unfinished x 3 = 15" (finishes to 13" )
The only ways to get 12.5" are to cut odd sizes, or use larger squares and cut down the whole block when you are done. If what you are making doesn't HAVE to be 12.5", why create a huge headache out of it, go a little smaller or larger! :)