If you can live with a 12 1/2" FINISHED block instead of a 12" finished block & you want to keep the proportion the same, this might help you.
Cut the square patches: 3"x3" (2 1/2" finished size)
Cut the Half Sq Triangle patch squares: 3 3/8"x3 3/8"
***2 1/2" + 7/8" = 3 3/8" (use your favorite method for constructing Half Sq Triangles)
Have fun!