Recube for Boss Damage?

I got these lines on my Fafnir Gun. I was aiming for 2 lines of boss damage, but would these lines provide a larger damage boost for me than 50%-70% boss damage? My understanding is that the 18% ATT potential would provide a 18% increase in damage since I don't have any other sources of % ATT. Is this a correct assumption? How much boss damage would I need on the gun to beat 18% attack and 35% Ignore Def?

These are my sources of boss damage, total damage, and Ignore Def (including Corsair skill updates from RED so Brainscrambler would be my main attack)

 Spoiler

Boss Damage: 210%
85% (Secondary Wepon)
50% (Fafnir Gun...will add 20% Boss neb eventually)
30% (RA Set Bonus)
15% (Demon Slayer Link Skill)
13% (Inner Ability)
3% (Demon Avenger Card)
3% (Kanna Card)
11% (Card Set)


Total Damage: 80%
20% (Jolly Roger - Skill)
10% (Demon Avenger Link Skill)
10% (Kanna Link Skill)
20% (Parrotargetting - Skill)
20% (Brainscrambler Reinforce Hyper)

Ignore Def:
40% (Brainscrambler Base)
35% (Potential Line)
30% (Crusader Codex)
20% (Fullmetal Jacket - Skill)
15% (Luminous Link Skill)
10% (Fafnir Gun)
10% (RA Hat)
8% (Pirate Card Set Bonus)
5% (RA Top)
5% (RA Pants)
5% (Avenger Title)