The formula for most champions goes> Time:CS>Deaths>Kills>Assists. There is possibly other factors but wards have nothing to do with it sadly.
So for each time bracket 20-25-30-35-40-55 there is a different number for kills needed and all that jazz to get an S. S+ you need to have ONLY 2-3 deaths just a heads up, some champions you can only have 1 death for an S+, can't remember which one that was, i think it was a mid lane assassin.
So yeah the hardest points to get a S or S+ is the 20 min mark, 40 and up. Those are incredibly hard to get S+ for.
The best way to gauge how much you need for a S or S+ is go to Champion.gg and check the general stats for the champion, see what most players get CS wise and kill wise. Then work on your play style from that to get S, otherwise just keep playing good.