I'll try to keep this short.

Armor Penetration's purpose was giving good early game damage and falling off significantly late game. Lethality is slightly better late (when it falls off anyways) but it's a lot weaker than armor penetration early.

The main problem with armor penetration was how effective it was when it was stacked in huge amounts. So instead of using the current formula used to calculate Lethality, I tried to find a function to keep armor penetration strong early while significantly reducing its efficiency if built in huge amounts.

I needed a function that didn't decrease low amounts of armor penetration but decreased high amounts by a lot. So i opted for Reduced Armor Penetration = x - x² / 200, where x is the amount of armor penetration gained from items, runes and masteries. This is the result

The function could be changed in so many ways, but that's what i worked with. The function starts descending after 100 armor penetration, which can't be reached anyways; even if it could, the function could be changed accordingly.

As we wanted, the function barely touches low amounts of armor penetration but significantly hurts stat-efficiency when you build lots of it (If you build 30 armor penetration, you lose 4.5; if you build 60, you lose 18).