First, make a list of your top-20 artists overall. Then, for each of these artists, add the 8 most similar artists to your list. Delete any duplicates, count up the number of entries on your list and this will give you some idea of how eclectic your listening habits are. A score of 9 represents an extremely unvaried musical taste while a 160 represents an extremely varied one (DO NOT count your top 20 artists, only the similar artists).
I don't know if this is a rule, but when my top 20 came up as similar artists, I did not include them. For instance, I didn't count The Beatles when they were listed as a similar artist for ELO.
Top 20 (At the moment)
Electric Light Orchestra
Great Big Sea