The word is it is around 15-30% of the sales generally. Mr Millar’s reveal of some of his royalty statements say foreign sales are a similar figure, around 30%.
So all the numbers are always much higher in reality than the Diamond estimates but you have to take into account it is comparative. If 15k is the cancellation point there most of the comics may be actually selling 25k but that’s if it follows the typical pattern.
Editors at Marvel have said that Ms Marvel, where the target audience is outside the typical crowd, is their first title ever to sell more digital copies than print. So the Diamond numbers there may not matter as much. Similarly for books that have a strong trade following.