Block and parry only work if you are autoattacking (or if you started a fight and your last attack type was autoattack and you haven't started attacking or shooting yet).
Edit:
Block works at half efficiency when autoshooting, so you will receive the bonus, but the penalty to block is applied after the bonus, effectively halving it.
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Computing the probability that at least one of the following events will occur:
P(a or b ... or z) = 1 - P(!a and !b ... and !z)
Probability
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