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That's also incorrect. Essentially, we have a 30 to 50% bonus from luck (there can be artifacts that are easy to obtain, and there isn't much else useful to put in their slot). Plus, there's elven luck (damage from successful attacks * 1.25).
That is, the modifiers are as follows:
Without Skill and artifacts: 1.375
With artifacts but without Skill: 1.625
With Skill: 2.25
The increase in average damage is 63.6% and 38.5%, respectively (because percentages should be calculated not from the base damage, but taking into account all bonuses, minus the Skill).
An additional advantage of Skill, besides counterattacks, is the stabilization of probability. Luck without Skill does not have enough predictability, which is not taken into account in the value of the average damage, but reduces its value.