To calculate Evade, all Evade-boosting bonuses are totaled multiplicatively. So, to calculate how much Evade your ship will have, you can use the following formula, where x, y, and z are percentage-based Evade bonuses that have been converted from a percentage to a decimal (i.e. 30% = .3):

Evade = 1 - [(1 - x) * (1-y) * (1-z)]

So, in the case of an R5 Stingray (30% Evade built-in) to which you're adding Guidance Scrambler III (40% Evade) and one Evade Upgrade (15% Evade), the formula would look like:

1 - [(1 - 0.3) * (1 - 0.4) * (1 - 0.15)] = 0.643, or 64.3% Evade

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Accuracy, on the other hand, is totaled additively, which is much easier to calculate. You can use the following formula, where x and y are percentage-based Accuracy bonuses that have been converted from a percentage to a decimal:

Accuracy = Base weapon accuracy * (1 + x + y)

So, in the case of a Siege Missile D55-E (50 Accuracy), which you're adding to an R5 Vindicator (+20% Accuracy) with the Missile System III special (+40% Penetrative Accuracy) equipped, the formula would look like:

50 * (1 + 0.2 + 0.4) = 80 Accuracy

Note here that *this allows Accuracy to exceed 100*; this is correct. However, an Accuracy greater than 100 will only matter when being used against an enemy with Evade.

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When calculating how Accuracy and Evade interact, the attacker's total Accuracy stat (which is treated as a percentage) and the inverse of the defender's total Evade stat (also a percentage) are converted to a decimal and multiplied together:

Chance of successful hit = Accuracy * (1 - Evade)

So, in the case of the above Vindicator (80 Accuracy) attacking the above Stingray (64.3% Evade), the formula would look like:

0.8 * (1 - 0.643) = 28.56% final hit chance

These formulas for Evade and Accuracy can be applied to understand the hit chance of any weapon in the game.