If I had to venture a guess, I'd say you cut on the sew line. Having never made one, I wouldn't know for sure...Maybe try again to make your diamonds, but add a 1/4". I'm thinking that your outside pieces on each half should meet at 1/4" from the end, and actually cross each other beyond that so that the points all meet together in the middle seam. I wonder if there is a block pattern online somewhere for this...now I have to look! lol