Formula to Figure Out Range?

Well, the damage formula is the same for all classes. That being:

(Weapon Multiplier) * (4 * Main Stat + Secondary Stat) * (Weapon Attack / 100)

First, you're going to need to give us the amount of attack you currently have (the total amount from all sources), that way we'll be able to figure out the weapon multiplier on Dual Bows (assuming it isn't already known), and then we'll be able to figure out the effect that tiara will have on your range.

Do you mean the about of WATK on everything? Dual Bowguns are 90, Maple Ring is 1, Silent Crusade Rings are 3 altogether, and Legend ring is 3.