Mark, the demarcation of how deep the root use to be will the white/green fade area of the leaf stalks, near the base. I'd plant them about that deep. Treat the existing leaf stock gently while handling them. IME, if you loose all the leaf stalks on a root to below the sand level, the newly placed root may not send up new shoots, even from a large surviving root. I also used FW plant tabs made by seachem when I first set the plants, set next to the new roots. I don't know if that helped.
The energy in Turtle grass seems to be stored in the thicker, horizontal tubers. The shoots I got that just were just a leaf stalk and small vertical root section snapped off the tuber did not survive very long.
Be prepared to give it time. I had one surving Manatee root send up a few new shoots in the same spot for almost a year. Now it is spreading rapidly. Grass in my tank grows towards the deepest sand.