I don't know Leto, but, my guess is it is predetermined as the rate per section varies from book to book. You start a book and recieve 9 simoleons (children's book) for each interim payment, the book is unlikely to be a best seller, but if you get 14 simoleons, the book probably will be a best seller. That is what I have observed, but, I could be wrong about the best seller/hit being fixed.