Any possible way to close the damage range in assasin?

Lucky 7 uses a special damage formula that ignores attack mastery. I don't think there's anyway to "close" the range. Just get more damage.

 

Luk mostly improves your max range while dex improves your min range (somewhat). If you feel like you need more stable dmg you can always add dex, but that will lower you dmg output since luk is your primary dmg stat. I'm just telling you this as information. Please don't go full dex.

Assassins always have unstable dmg, but in return their average dmg output is one of the best in the game.

 

DEX has no effect on lucky 7 damage, and I believe by extension Triple Throw damage as well. So you're stuck with the variance you're getting.