What do i need to get an S?

Can you please just tell me what are your rating's based on? because i just did Chogath 18-3-16 and i did not must damage but still i carry my team and got like a dozen kills in row its just too ridiculous your ratings do i need an perfect flawless game to get an S? i mean come on ive gotten with worst score an S and i got with better also but still 18 KILLS 3 DEATHS and 16 ASSISTS {{summoner:14}} SERIOUS?! {{summoner:11}} REALLY?! i mean your rating system sucks pretty damn much...got better gaming to do than be here asking for an answer about something you really dont know or dont care is players having good games carrying doing 18-3-16 and Still getting an A+ is fare for you guys...{{summoner:3}} {{summoner:4}} http://matchhistory.euw.leagueoflegends.com/en/#match-details/EUW1/3251639664/36324155?tab=overview
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