I'm sure there is a 'rule' out there but hand towels come in different sizes so personally, I'd look at the towel/size and judge from there. If you want a larger size, place lower to the edge of the towel. A smaller size font or design would be up a bit. I'd fold the towel in half like it would be if hung on a towel holder and visually decide what would be the best size. I think visually appealing is more important than what is standard.

Lori

Lori