I believe it would come out to 11-1/4 finished size. Her's was 12-3/4 before the cut into 1/3's (each section would have 1/4 on each side, so that would total 1").
Some blocks (especially the disappearing ones) just don't add up to a perfect 12-1/2" size. You could experiment with different size blocks and see if you can come close. If you are doing a whole quilt with this block, you could just make as is and adjust the border size to get the overall size you want.
You can always try making them out of paper so you don't waste fabric experimenting.