It’s in the combat blueprint ofc… the one that’s like 20 MB of code…
here’s a picture…
That penetration function returns your armorpen subtracted from 1… do you want a picture of that too? 
That’s what comes out of the Get Armor Penetration function in the picture above
Edit: If it helps I even updated my devkit in between my original post and this… and nothing changed… so it’s going to be the same for chapter 2 as well…