For marksmen the only thing that is required is 100 cs for every 10 minutes of game time.
I had 12 deaths in one game but had like 350 cs 40 minutes into that game and got an S-
So dumb. They don't even factor in damage to objectives.
It's also relative to how well other players do with that champion in that role. By rule the 100 cs for every 10 minutes of game time is the best way because that's near perfect cs. However if you take Kai'sa top or mid, it will compare your performance to all the Kai'sa top or mid games, which means it could be easier to get that S playing a champion in an off role (at the risk of possibly losing).
After laning phase it can be hard to get the cs goal because once you group up no other player just lets you have all the cs in the wave, if they are nearby they will not let you have it because they are greedy little turds that don't understand that marksmen have the most expensive and ridiculous build path of all the classes (god forbid you take one jungle camp or red buff late game from your jungler, holy shit here comes the flame train). And if your marksmen can't buy items your team is fucked.
If you're fed it can be even harder to get that goal because at that point you don't really need the cs anymore cause you can just kill all the enemies already so it's definitely not based on whether or not you carried the game.