Just a question about % bonus armor penetration and armor cleaving

So when you get % bonus armor pen, as it says it penetrates in function of the bonus armor of your opponent. But what about when your opponent's armor is getting cleaved? (reduced) How does it work? Does it still penetrate the same value like the cleave never existed, does it on the opposite count as the cleave affected the bonus armor first, making sometimes bonus armor % pen absolutely worthless? (example : Guy has 50 bonus armor, I cleave 50 due to black cleaver or sthg like Wukong Q or Nasus E ; if I have a is the value of pen null? Or 3rd option which would be weird, is the cleave equally divided between base armor and bonus armor and then calculations of pen are done? (I doubt so)

Any guy here that know exactly how it works, or a Rioter that knows it? Thank you very much, for the purposes of theorycrafting.