How Good is Lethality?

So, this preseason, we're getting a new stat in place of armor penetration: Lethality. There are 3 items that have 20 Lethality each on a unique passive. That's a possible 60 Lethality, which we know will scale with the target's level. I've heard that the math on it is 40% Lethality is basically Flat Armor Penetration, and the other 60% will be added in based on the target's level. That's a total of 24 Lethality that's outright flat and 36 that's being added in at a rate of 2 per level when you have the full 60. I'll be using the stats of champions who are primary role Marksman to assess how strong this is, since they will more often than not be the assassin's targets. All of the math is from their pages on the League of Legends website, and I'm using the simple formula of b+(s*18) for this, where b = base armor and s = armor per level. First, I'll list each champion's base armor at level 18, then tell you just how strong I think this new stat is.

Ashe: 82.412 Caitlyn: 85.88 Corki: 86.38 Draven: 84.944 Ezreal: 84.88 Graves: 85.576 Jhin: 83 Jinx: 85.88 Kalista: 82.012 Kindred: 83 Kog'Maw: 82.88 Lucian: 78.04 Miss Fortune: 78.04 Quinn: 86.38 Sivir: 80.71 Teemo: 91.8 Tristana: 76 Twitch: 77.04 Urgot: 83.944 Varus: 84.412 Vayne: 80.212

The highest base armor here is Teemo's 91.8, but if we don't count him because he isn't played as an ADC, it becomes Corki/Quinn tied at 86.38. The lowest is Tristana's 76. Subtracting 60 from each of these leaves us with 31.8, 26.38, and 16 for the range of base armor values after 60 Lethality is factored in. I can pretty easily see ADC champions building at least 1 armor item when against an AD assassin next season, but overall it appears to be a fairly balanced stat that does what it's supposed to do: help assassins scale into the late game without making it busted in the early game. Thanks for reading and have a great day.