Copy pasting skills!
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".
Silentium - 2,560
Regicide - 988
Beseech - 4,878
Aina - 6,141
Epica - 11,919
Ayreon - 26,911
Avantasia - 20,416
The 69 Eyes - 29,433
Xandria - 6,057
Poets of the Fall - 18,539
= 127,842/10 = 12784.2
Radiohead…