Armor usefulness calculation?

[Samseng Yik]

In the past I thought 1 armor = 1% reduction.
Then there is 1 round I stack 200 armor, still receive damage.
So I just want know what is the calculation for armor damage mitigation.
Is it 10 armor = 2 damage flat reduction ?

[Xuande]

There's a Steam guide that claims "(ARMOUR x 100) / (100 + ARMOUR)", essentially 1 = 1% with diminishing returns.

[Lord Agamemnon]

While each point of armor is diminishing returns to damage reduction, it's constant returns to your effective HP.
For example (you probably won't see numbers this big, but it makes it easier to show the math):
With 0 armor, you have 0% reduction. If you have 100 HP, it will take 100 damage to kill you.
With 100 armor, you have (10000/200) = 50% reduction. If you have 100 HP, it will take 200 damage to kill you, so the first 100 points of armor give 100 effective HP.
With 200 armor, you have (20000/300) = 66.666...% reduction. If you have 100 HP, it will take 300 damage to kill you, so those points of armor gave you another 100 effective HP.

[Xuande]

I'm a recent buyer with the current sale, so I'm unaware if its truly EHP due to lacking metaprogression unlocks (so I don't know what does and doesn't bypass Armour), but this particular damage mitigation formula is a popular one used in many games.

I use a variation of it myself in a game project of mine, for offense and defense both - Base Damage * ([Offensive Stat + 100] / [Defensive Stat + 100]) - as its good for preventing overpowered outliers from a stat being extremely high or low, without having to put a hard cap on said stat.

[Samseng Yik]

The current calculation is good enough.
I am satisfy.