For a 6" finished block you want a 6-1/2" block. If you are drawing a diagonal line and sewing 1/4" on each side of the line, you want to add 7/8" to the block size (6-1/2 + 7/8) if your 1'4" seams are accurate. I find it better (not only for accuracy but mathematically) to add 1" and then trim to the desired size. That means for a 6-1/2" block you will cut 7-1/2" squares.