Cool to see people in these forums still interested on challenging this game! I'll bring some useful information to you about S-Rank calculation:
PB DMC4 Forums
This can clarify things when you don't have one idea on what you have to work on to get the S on some missions. But most of the time it's a very straightforward thing to improve what the rank shows you that you not did so well, but sometimes it's cool to think about another strategys to make that S come out. In DMC4 i'm generally more annoyed with the orb rank than anything else, sometimes it forces you to find some hidden red orbs, for example.
This playlist on YouTube can be quite useful for you too: