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From what I read somewhere (can't find it anymore):
Healer: Power comes heavily from defense value
Tank: Power comes a bit more from defense value than offense value
Melee: Power comes a bit more from offense value than defense value
Ranged: Power comes heavily from offense value
Whatever slot your main character is in counts as a 20% boost.
The % bonus for non-main characters is believed to come from a particular stat of the fleet member in that slot:
Ranged - Base attack
Tank - Base defense
Healer - Base energy
Melee - Base stamina
Feel free to correct me if I got any of that wrong.