My take on Arcane Bolt is simple--having the base damage according to the formula is fine, and having the bonus damage tied to a gimmick is fine--but it shouldn't really be random, it seems like a missed opportunity for specific interactions.
Perhaps, as it has a decent chance for knock back, have the bonus damaged tied to only affect enemies that are currently knocked down, for example.
Something like that might even make it more useful for melee characters, as a followup for knockout blow, etc.