Basically: DMG = DMG - ARMOR.

More depth: 
Maximum after-armor damage is always 50%. This portion of damage is mitigated at full amount of remaining armor(240 armor remaining? ok, then it will mitigate 240 points of after-armor damage).
50% armor ignore means this 240 armor will be mitigating after-armor damage as 120 armor.
100 armor penetration means this 240 armor will be mitigating after-armor damage as 140 armor.
At least 10% of damage will penetrate armor, as long as skill is not 'cannot penetrate armor'.

Formula is like:
damage = damage - mitigation
damage to hp cannot be lower than 10%
if armor wasn't destroyed: damage to hp cannot be higher than 50%