Quick affix question

[Zepia]

Hey there, so I recently got the full Vardha Unit set and since it looks like I'll be using these for a long time, I'm going to be affixing things to it.
At the moment each of my Units gives +30 R-Atk, two has mutation and vinclum, and one has fang soul.
My question, is an additional +90 R-Atk worth it? (Shoot 3 + Soul)
How much damage does it really add? I currently have 1594 R-Atk, and a friend said if I normally do 1000 damage, it would add 50-100. So is the extra 90 R-Atk translates to +10% or so damage?

[UnLucky]

Well, if you go for 3 slots, you can get a good amount of HP/PP as well