I think im missing a point with Magic pen
if I get sorc shoes, void staff and haunting guise
is that not 80% magic pen?
There are 3 Different types of Penetration.
1. Reductions. These reduce armor/mres by a set amount and affect everyone who attacks them (starks, abyssal, fiddle's aura, nasus's spirit fire, etc). 20 reduction reduces armor by 20, ergo, 0 becomes -20, 20 becomes 0, 100 becomes 80.
2. Flat Penetration. These penetrate a flat amount of armor/mres, and thusly only affect the owner of the penetration (haunting guise, sorc shoes, brutalizer, olaf's ult's passive, all penetration runes, sunder mastery). Again, no % here, it works the same as reduction except you can't penetrate to below 0.
3. % Penetration. Same as flat penetration, but it's a %. Very few sources of this (void staff, archaic knowledge mastery, last whisper). This is applied last in the order of penetration.
example, you are fiddle with sorc shoes and void staff.
vs. 0 mres target:
-10 from passive = -10
20 penetration = -10 (cannot penetrate below 0)
40% penetration = -10 effective mres(cannot penetrate below 0)
vs. 30 mres target:
-10 from passive = 20
20 penetration = 0
40% penetration = 0 effective mres
vs. 80 mres target:
-10 from passive = 70
20 penetration = 50
40% penetration = 30 effective mres
vs. 200 mres target:
-10 from passive = 190
20 penetration = 170
40% penetration = 102 effective mres