Heroic Pere AA Damage Issue

[Monarsi]

Arc ability says it adds %50 damage (stacking). But according to my calculations its much more lower than this. Especially when ur damage goes +20k It becomes %15-25 damage.

I even tried it for low damage (5 dmg). It adds nearly %40 dmg.

Maybe there's something I don't know about damage stack. I would appreciate it if someone who knows that can enlighten me.