% Penetration, Armor, and MR

Penetration and tank itemization (tank stats) have become a common discussion topic.

I want to take the time to detail % penetration, just lay out numbers and try and show why %penetration, and penetration in general, is obscenely strong.

I'll use Last Whisper and Void Staff's % pen values (35%) and use that as a base for this. The following numbers represent MR/Armor before penetration, after penetration, and how much each mitigates pre/post penetration.

50 (.33), 32.5 (.25) 100 (.5), 65 (.39) 150 (.6), 97.5 (.49) 200 (.667), 130 (.57) 250 (.71), 162.5 (.62) 300 (.75), 195 (.66)

As you can see, 35% penetration removes almost 18 armor/MR from a squishy, and over 100 from a dedicated tank.

I though Flat Penetration was supposed to be stronger against squishies, while %pen was more effective vs. Tanks. That's demonstrably not true! %pen removes almost the exact same quantity of mitigation on tanks as it does squishies! (8%-11%)

This is a problem. This shows that % pen is actually MORE effective against squishies than tanks, and that's even before we apply flat penetration.

To put this another way, Last Whisper/Void Staff are 8-11% more damage, against everyone, for 2300 gold. Throw in another 1500 or so for Haunting Visage, Sorceror's Shoes, or Brutalizer, and you're removing another 10-15 armor/mr (50(.33), 32.5 (.25), 22.5(.18), 17.5 (.14)) and another 7%-11% mitigation, if not outright removing all mitigation entirely!

At the same time, tanks aren't feeling that. That 10-15 more penetration isn't hurting them that much: Flat Pen works as intended, it hurts squishies but only tickles tanks.

I don't know if this is intended or is an actual issue, but to be honest I don't think % penetration should be penalizing squishies harder than tanks.

I'm honestly quite suprised that that's the case. I thought I'd be writing a rant about how broken pen is vs tanks, when in fact it's utterly broken against squishies too, if not more so!