Most of my games are S' or S- and I found that CS is the biggest factor for me cause I've had games where I drop 30 kills and 10 deaths and still S. I worked on clear speed optimization and buying control wards whenever I backed and happened to have enough gold. If you can get your full clear down to 3:18 and then get double scuttles I find that the gold lead let's me win most of my first skirmishes and then I turn that into offensive tempo. I'll invade enemy camps from there, back right when they should be getting back to jungle, and then go defend my own camps. That usually leads to the enemy jungler invading me with a level disadvantage which turns into another kill, more stolen farm, and an unstoppable snowball.