Lethality Graph with Items
This is the second part about Lethality. See this link for the first part: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/nUw7wuM0-lethality-for-challenjours
Now to purchasing Lethality Items. I came up with descent levels that the Lethality Items 
are purchased.
http://imgur.com/a/wzvpQ
The first chart below includes the Precsion Mastery, the second one does not. The chart to the left looks at lethality marks and quints, the one to the right looks at lethality marks only.
Now the analysis: In every case, Lethality is weaker pre 14. However, at level 14, Lethality gains the large upper hand. Since the average ADC has 88.4 armor from runes and base armor, you can get very close to dealing true damage if you run full lethality marks and quints (82.5 armor penetration at level 18). On top of that, if you get something like a 
, you can easily ignore your targets armor.
So how much of a nerf is Lethality? For lethality marks and quints, its about a loss of 9 armor penetration every time you pick up a new item that increases your lethality prior to level 11. When you are not buying more lethality, then that value drops to a loss of 6-7 armor penetration, until it goes back up to 9 after purchasing another lethality item. After the enemy reaches level 11, it is closer to 4, until they even out near level 13 ish. However, you gain 15 armor penetration at level 18.
For lethality marks, its about a loss of 8-10 armor penetration every time you pick up a new item that increases your lethality prior to level 10. When you are not buying more lethality, then that value drops to a loss of 6-8 armor penetration, until it goes back up to 8-10 after purchasing another lethality item. After the enemy reaches level 11, it is closer to 4, until they even out near level 13 ish. However, you gain 12 armor penetration at level 18.
So should you take lethality runes? It depends on your play style and if you think that a loss of 8 penetration pre 11 is worth the potential gain of 15 armor penetration at level 18. Will I be taking it? Of course.
