To get a 12.5 unfinished block size you need to start out with squares that measure 4.3333333333 inches. When you sew the first three squares together they have to measure 13 inches and then you cut and take out another 1/2 inch seam allowance.
The solution is to either make them slightly bigger or slightly smaller or be happy with the size they turn out.
My bad, it's 4.833333 for the cut size and 4.3333333 for the finished size.