Eh, I'm not a fan of martingales by any means, but that one looks like it's adjusted properly. I was always taught that you want the rings to hit a the point of the horse's shoulder, any higher than that and you pretty much defeat the purpose of even using one. Also, I believe the point is more to get the horse to tuck his nose in and round up, rather than to encourage a low headset. Again, I don't like to ride in the things but that's just what I've picked up from having to use them on the school horses in college.
Also I would like to point out that many western pleasure horses are bred to carry a naturally level headset. My mare (has cow horse bloodlines but still) almost always travels with a level top line, not because I've forced her into that position from gadgets or other training methods, but because that's the way she naturally carries herself.