The actual equation is something like this (simplified).

(((A + B) - C) * (1 + D / 100) + E * 0.1) + F

A = Attacker's Hit Rate
B = Attacker's Skill Rate (Every skill has a rate modifier, most are set to around 80% but some are 100%).
C = Target's Avoidance
D = Skill success modifier % (Some skills have a bonus hit modifier).
E = Skill success modifier + (Some skills have a bonus hit modifier).
F = Attacker level below target: (((target level - attacker level) * 1.8) * 1.3) (capped at -12)
Attacker level the same or above target: ((attacker level - target level) * 2) (capped at 12)