Either do some math, or use just refer to some decent but generalized guidelines.
Generalized Guidelines: 50%pierce for norm, 60% for NM and 80% for Hell.
Math: Consider the rate at which your damage changes when it is reduced by positive monster resistances, buffed by negative monster resistance, and buffed by +%SkillDmg. Find the best value of -%Res and +%Dmg. In general, your +%SkillDmg should be the same as the negative resistances of the enemies to maximize your damage (i.e. after you've got -80%Res, start considering +%SkillDmg (e.g. -100%Res with +20%SkillDmg is a good idea)). Sorcs only need to consider -%Res since their Masteries apply so much +%SkillDmg that they are excessive.
Average Monster Res in HUv1.21z was 50% norm, 60% NM and 80% Hell.