The Basic Guide to 'When Should I gem for Armor Penetration?'
Due to how Armor Penetration works, an exponential increase scaling with itself and your base damage, ArP is a stat that will eventually out-perform Strength until you cap it.
The three main factors determining how good ArP is for Blood DPS:
* Current Weapon
* Current Strength
* Current Armor Penetration
Once you hit a certain level of Str and ArP (With the right weapon), then you'll be at the point where you can ditch those Strength gems for ArP gems.
Here's a rough estimate for users of either Cryptmaker or Shadow's Edge.
* Weapon: 3.50+ speed, ilvl 264 or greater
* Strength: 2,200
* Armor Pen: 400
* 4pT10 set bonus
Once you're at these numbers, you can safely gem for ArP and see a small increase in DPS. While it starts small, the gain grows more and more with the more upgrades you attain. Your ArP does not have to be capped for it to be a DPS gain.
Users without 4pT10 or a high end weapon will find the Str/ArP required to be much higher and it's not recommended to try ArP gems until you meet the requirements listed above.