I read somewhere (tool tip I think, but could have been the manual or loading screen tip) that the maximum bonus / reduction you can get is 3 times.
Dunno the exact way it's calculated, but I'd imagine it's attack/defense. Just working it out on 3% per point would mean very little difference between low level armies (eg. swordsman and a fairy).
Eg: your enemy has 10 defense.
You have 1, 2 or 3 attack, you do 33% damage.
You have 4 attack, you do 40% damage.
You have 10 attack, you do 100% damage.
You have 20 attack, you do 200% damage.
You have 30 attack, you do 300% damage.
You have 31 or more attack, you still do 300% damage.
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