u/Even_Complex5665

I don't know how others tried to test this, but here is an example for what I did.

You can clearly see an increase in damage going from 7.5% Magic Pen to 22.7%
I also tested what it would be switching those gear pieces to give Fervor instead
Just be very careful that none of your other stats change, like in this example my Faith number remains exactly the same for each test.

I have tested this with regular Armor Pen on physical damage too, and results appear the same.

Level 20 Bosses have 1,500 Armor (physical and magic) using the Formula: Damage Reduction % = - Armor / (Armor + 2285)
You can ignore a percent of this Armor value with your Penetration
Every 6 points of Penetration = 1%
At 600 points you will have 100% which should be the cap allowing you to ignore all enemy armor
keep in mind of other skill effects that debuff the enemy, reducing their Armor/Magic resistance, they will directly reduce this armor value on your target

For a full overview on how the stats contribute to damage and how to properly test this yourself, check my video here: https://youtu.be/zcIMQjmQXlI
And also you can make use of my Formula document here: https://docs.google.com/spreadsheets/d/18Lh1FykoJL4Q1QjVW32h0eQCaCt1w-NBNyW8K2E_mIM/edit?usp=sharing