For me, this is the rough way I score. It's how I've scored songs since I was very young & my score basically equates to a U.S. school grade mark.
A: 9 -10 = A song that I really enjoy & would playlist (either if I made an artist specific playlist or some themed compilation.)
B: 8 - 8.9 = A song that I would probably not skip if listening to it on album, playlist, etc. (but I would probably not personally playlist it)
C: 7 - 7.9 = It's fine, but not something I would ever come back to & would likely skip if not rating.
D: 6 - 6.9 = Doesn't work for me at all, but not a complete failure of a song, I guess.
F: 5.9 & under - various degrees of complete failure. (To be fair, I seldom rank songs under a 3 & I am personally not a fan of zeros... but I understand many people use the whole scale, etc.)