Original Kudos/Hits and Bookmarks/Hits ratios were inherently flawed: Bookmarks/Kudos accurately represents 'The number of people who LOVED a fic/The number of people who LIKED a fic'. A thread detailing the accuracy of this tool can be found here
1. It's not how 'Good' the fic is. Think of it as the ratio of people who liked it, to the people who REALLY liked it.
2. To build on that, if you LOVE College AU's, and the percentage says 40% on a college AU, you will probably love it. If you hate mobster AU's, and one has 40%, chances are you will still not like the fic, because you don't like mob AU's.
3. Each person's rating of what a 'good' fic is different. You can't numerically quantify how 'good' a fic is. I'm a picky bitch with a low tolerance, so I won't read anything under 27%. You might go higher - 30% is your lowest, or lower, and enjoy anything higher than 20%.
4. The tool won't work properly if you sort by recent posts, because the number of bookmarks on a fic, the less accurate the percentage will be. This also goes for smaller fandoms.
5. 'Bookmarks' = private or public bookmarks or recs, meaning:
1. Rare Pairs might have higher ratings, because they're starved for content and will rec anything
2. PWP or ABO fics will have lower ratings because people don't want to broadcast the fact that they read werewolf porn.