Skill Tooltip Damage

The Skill Tooltip Damage can be found by hovering over a skill in your ability tab. It has a minimum and maximum value, representing the actual range of damage for the skill after Bonus Damage is added to the Base Damage range.

Equation
The exact equation for the min and max Tooltip Damage varies between skills due to the different skill multipliers. To get the full equation for a particular skill's Base Tooltip Damage, find the skill's multiplier in the Skill Multiplier list and plug it into the equations below:

Minimum: Minimum Base Damage * Skill Multiplier Maximum: (Maximum Base Damage + Bonus Damage) * Skill Multiplier

Note that this equation may not match what your game displays because it is the damage before extra interactions via Talents, Symbols, and Equipment such as Amulets and Weapons have been applied.

Findings
Given the above equations and the equations for Maximum Base Damage and Bonus Damage, the most effective way to increase Skill Tooltip Damage on paper is simply to invest in Strength over Valor due to the exponential returns of Accuracy in the Strength/Accuracy interaction.

However, due to Temper's chance of applying the full bonus damage on hit, the Skill Tooltip Damage number on paper is misleading; if one were to taken and average of in-game non-critical damage of a skill, they would find it to be in the upper-half of the range given by the skill's tooltip.

That said, players would probably find investing in Luck/Critical Chance to yield more damage: Critical Chance and Temper chance presumably provide the same percent chance to proc, while Luck would provide over 1.5X more damage on the proc than Valor at 100% Temper, and 3X more damage at 0% Temper. Luck furthermore has the higher possible rate of damage scaling due to the way Temper and the Mobilization talent both contribute to the 100% effective Temper cap in the Bonus Damage equation, while Maximum Recoil is the only contributor to the 100% cap in the Critical Damage equation.

Skill Multiplier List
(last updated 7/23/2015)

Archer (Last updated July 24, 2015)