The way armour scales doesn't work like that, flat armour reduction is equally valuable against high armour targets as it is low. A tank that only takes 100 damage may have it's damage taken doubled to 200, still not much but the damage taken is double. A cloth wearer may have it's damage taken doubled aswell from 500 to 1000, they'll both die twice as fast if you reduce their armour by that flat amount.

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A good way to look at this is 90% reduction, a person is only taking 10% of the damage they'd normally take, if you reduce their armour by just 10% then suddenly they're taking an additional 10% damage, ie 20% which is double what it was initially. Similarly if someone has 50% armour then you're doing 50% damage, if you remove all their rest of their armour then they take the full 100% damage, which is also double.