Thanks for your suggestion. The thing is, I don't even know half of the songs and most of the other half I don't really want to know how to play (except for bands like Iron Maiden, Megadeth, SOAD and such), that's why I started looking for customs. :)
I'm struggling with this too... I've just started learning guitar and CDLC even with 8 difficulty levels starting right at 8 is by no stretch of imagination helping me learn. And also I've only played about 3 or 4 songs and got max to 40%, so there is no way it could be dependent on your mastery level in the game.