On 11/20/2015, a tweet revealed a bug that prevented destroyers from being able to hit the PT Imp Groups. As a result the devs readjusted the damage calculations in a live patch update on the same day.
[math]\displaystyle{ \text{Acc}_\text{Atk} }[/math] the standard accuracy described here
PT boats are "DDs" in the game, so the [math]\displaystyle{ \text{Mod}_\text{formation} }[/math] for vanguard is 1.1 during shelling, and 1.2 during the torpedo phase.
Historical accuracy bonuses during Events are included in the Standard Accuracy Term, i.e. it is affected by the [math]\displaystyle{ \text{Mod}_\text{PT} }[/math] modifier.
[math]\displaystyle{ \text{EVA}_\text{PT} }[/math] the PT estimated evasion described bellow,
[math]\displaystyle{ \text{Mod}_\text{Ship} }[/math] the bonus given by ship types, described below,
[math]\displaystyle{ \text{Mod}_\text{EquipmentAcc} }[/math] the bonuses given by equipment, described below,
[math]\displaystyle{ \text{Mod}_\text{Night} }[/math] being 0.7 during night battle, 1 during day battle.
Amagiri Kai Ni/D has the ability to prioritize focusing on attacking PT imps with significantly increased accuracy if any are present.
DD placed in the composition slots above and below her will gain a noticeable accuracy boost and will prioritize attacking PT boats if any are present.
The PT boat targeting rate is 100% for all affected ships [9].
During Events, some special bonuses may be added, with "historical" ships and equipment gaining some accuracy bonuses[10].