Quality is based in the next points:
1. If it is a soundboard recording or an audience, even in this point the sound quality is different from each one.
2. If it is a partial or complete recording from the show.
3. The quality of the performance
4. The number of distortions and other mistakes
5. My personal mood the day of the review
6. Others opinion
After all you can check your rates with mine to give you a clear idea or you can read the description of each one, also think about the next for me the worst bootleg in my collection is Maiden Voyage and is a 4 so take that one as a base.