I only have complete albums, which makes it a little tricky. Some albums *glares at David Bowie and Tom Petty, two terrible offenders* suffer from massive amounts of filler. Others don't. Anyway, I keep all the songs, no matter how lousy.
0 Stars -- means it's unrated, natch.
1 Star -- everything is at least this high.
2 Stars -- In the best 75% of my music. (It's much better for me to use numbers and logic to quantify my appreciation for a song. It keeps the ratings useful and the ratios apt)
3 Stars -- In the top half of my music.
4 Stars -- In the top 25%.
5 Stars -- In the top 10%.
Also, I do an album rating. For this, I add together the ratings of all the songs on the album and divide by the track number. Simple, no? I then multiply the number by ten and put it in the BPM field (ie, 45 = 4.5 stars). Theoretically, this would yield a 0-5 scale, but it really yields a 0-4 scale. Still, only a few bands (Nirvana, Leonard Cohen, Pink Floyd, Velvet Underground, etc) are in the 4-star category. The rest of them are distributed much like the song ratings -- 10, 25, 50, 75%. Might seem brutal for me to have a lot of 1-star albums, but I'm a pretty harsh critic and I have a lot of music 🙂
Of course, this system will likely change, at least in ratios. I'm rebuilding my library from scratch right now with lossless, good album art, meticulous ratings, etc. Mostly what I have right now are my favorite albums of all time as well as those that are just lying around -- so these ratios are pretty skewed no doubt. Instead of 10, 25, 50, 75, I'd like to have something like 5, 10, 20, 40, 80.