I always have trouble getting a sawtooth block with its flying geese pieces to come out a perfect 12-1/2" block. These geese make me nuts!!! So I cheat. The center is cut the standard 6-1/2" square for finished 6" center. The sawtooth points are cut at 3-1/2" squares as normally done. But I cut my corner and pieces for the flying geese bigger. To make a 12-1/2" square, the normal directions say to cut these pieces at 3-1/2" squares for the corner pieces, and 6-1/2" x 3-1/2" for the flying geese pieces.
I cut these pieces at least 1/4" wider, usually 1/2" wider to make sure I have plenty of room. You can see in the top block how uneven the edges are. But when you press it and center the square on the block, you get a perfect block with the correct 1/4" seam allowance.
P.S. As you can see, I love scrappy quilts!