Go through your top 10 artists and add up the total number of listeners. Divide that number by 10; this is the average number of listeners for your top ten. Now divide the average by the number of Radiohead fans (Radiohead is the artists with the most listeners). Multiply the result by 100–that is your "mainstream percentage".
Barenaked Ladies - 119,225
Tegan and Sara - 124,741
Ani DiFranco - 76,100
Blink-182 - 335,948
Paul Simon - 102,184
Ryan Adams - 112,732
Fountains of Wayne - 73,891
Simon and Garfunkel - 255,898
Me First and the Gimme Gimmes - 76,724
*Nsync…