At 120 Chivalry with EoO and Elemental Slayer (which is a super slayer) you do your Displayed Damage x 2.82 (or x 2.97 if Consecrete Weapon because of the additional 15%) , minus the target's resistances (if not AI).
Your Displayed Damage (in the stats gump) is likely much more than 10-20 since it does include your DI100, your Tactics bonus, your Anatomy Bonus and your STR bonus.
Note that if you just fight Unbounded EVs, an Air Elemental Slayer would allow you to reach the x3 cap without even using EoO (and you wouldn't take extra damage from the other mobs)
Now while I agree that when you do a lot of damage, a bit more or less may not seem much of a big deal, remember that it also ends up affecting what you leech. It is especially true for range weapons where Mana and Life leeches are halved.