Champion Mastery in Poro King/Aram modes

So while I have no info based on how champion mastery is calculated exactly, I feel that the metrics are the same on SR as on Howling abyss. Which leads to the scenario that it is nearly impossible to get an S rank on HA simply because of how its calculated (cs, deaths etc.). I mean if I can go 15/3/19 (on jhin) have the most cs on the team, nearly 80% kill participation, and double the rest of my teams damage and still only get a B+. Well then I would say with fair certainty that its nearly impossible to get S. That's just my thoughts what about you guys were any of you able to get an S in poro king?