I don't know much about RNG, But i think Arching Arrow is the only Critical hit Ranged WS?

Yah, I'm sorry i can't help much, I don't know the best Atmas for RNG, I would have guessed VV/GH/something but yah, shows how much i know about RNG.

Go shoot some Colibri in Abyssea-Mis and tell me if those seem like familiar numbers.