I did a post on armor reduction in another thread:

http://www.spacetimestudios.com/show...02#post1206602

In my post, I have a public Google Doc Spreadsheet, where all you have to do is enter your HP, the DMG from an enemy/boss and the mystery multiplier that STS has based on your level and mob level.

Play around with the spreadsheet and you can see that additional armor has diminishing returns when you have 4.5k HP and 1400+ armor.

The difference (as percent of total hp pool) is 28 HP or 0.5% of your total HP from 1000 DMG between 1500 and 1600 armor.