Go through your top 10 artists and add up the total number of listeners. Divide the average by the number of Radiohead fans (Radiohead is the artists with the most listeners). That is your "mainstream percentage".
Red Hot Chili Peppers:516,952
The Go! Team:112,254
Budget of the Shopping Anarchist:28Bob Evans:1,606
Obscurely Mainstream,not sure if that makes sense.