So I was just in a game and actually took the time to look at how much damage I was doing to players as AP sion.

I had 49% MR pen with 51 flat pen. % based is supposed to go on first, and if that really is the case I should be doing pure damage up to around 100 MR. But I wasnt. I was doing pure damage to 51 and the it slowly started decreasing as I hit targets with more MR.

A target with 114 MR took ~420 damage from my shield with ~220 AP (it shouldve done ~540). This would be consistent with % based mpen going on after flat.

114 - 51 = 63
63 x .51 = 32.13
100/132.13 = 75%

He took (based on the rough numbers I dont remember the exact ammount of AP I had or damage he took.) 77% of the damage he should have which is pretty much the numbers you get if you put % based on after.

Can anyone else confirm that % based penetration goes on after flat now? This would mean a huge change to the way I play casters.

%-based penetration applies AFTER flat. This has been true for... 6-8 weeks now, I think. Maybe longer.

EDIT: Just for reference, here's the patch where it happened.

http://www.leagueoflegends.com/board...d.php?t=125023

Wow well **** me. When did that change go through? Who decided it was a good idea to gimp % based pen?

It was changed to help prevent people from doing close to true damage. It affects both Magic and Physical penetration.

Essentially, you either need 20-40 flat MPen or a Void Staff, plus runes/mastery imho. I tend to go the flat route, myself, since my targets often don't bother with much MR.