To do this yourself, use your top 20 artists, and look at the eight highest artists on their similar artists. If any of them repeat, do not count the second entry and every entry after that. If any similar artists are also in your top 20, do not count them either. If any entries are the same artist but with a different name, don't include this similar artist, pick the next one down. For example, in Michael Jackson's similar artists both 'The Jackson 5' and 'The Jacksons' are included, so you would ignore the 'The Jacksons' and pick the artist below them in the similar artist list. It sounds complicated, but it's easy once you get started. Once you have your list, count up every first entry of an artist. Your final socre will be out of 160, so the highest score you can get is 160, which indicates extreme variety. If you want, you can just do it for the top 10, in which case a score of 80 represents extreme variety.
1. MuseFranz Ferdinand
30 Seconds to MarsBiffy Clyro