here's a little damage graph i came up with
based on having a lvl 12 conviction aura active
x axis is enemy lit res
y axis is lit dmg
the 15,20,.. are how many +skills
the -1,-2,-3 refer to how much -enemy resists your gear has in gaps of 20 (1 refers to 0 -enemy res, 2 refers to -20% enemy resist and so forth)
there is some information or scenarios left out because they are not likely possible
the damage lines are just a shift for +skills and can be interpolated in between if you know how much -enemy res you have from gear