Mastery System

Feel as if the way mastery system works being based on the player base is terrible, scores are extremely ridiculous for certain champions like lucian so far i've been trying to get a mastery 7 and the ONLY S i got was from being 20/4 and not an S+ a S. There should be a set requirement for all champions and roles at certain time frames such as when I did jungle lucian 8/1 by 20 minutes not even an S- and when a champion is nerfed the previous player ranked scores are also still there making it almost near impossible to reach the required KD