I drew out a 12x12 block on graph paper. The middle strips come out to be WITHOUT seam allowances to be 2 and 13/16ths by 8.5. You can always round that to 2 and 7/8th and trim down a bit. So I would cut my middle strips to 3.5 by 9, sew them together and trim down if needed. This block doesn't seem to be an exact science. The triangles would be 6 and 7/8th as you stated before. Try one in scraps and see how it goes.
Oh Courtney, we were both on the same track!!