Your math is right for the gem bonus, but "respectively" in the description refers to melee and pet, not the two targets. Melee hits will deal 52% normal damage to both targets, pets will deal 48% normal damage to both targets.
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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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