What this spreadsheets shows is how many books you’ll need to buy before you start to save money from coins.
This sheet doesn’t go over how much you’ll save by not having to pre-order a book to get access to the pre-pub.
If you are buying a lot of Books from JNovel Club their Premium Tier is fantastic, especially if you are new to AOB and want a cheap way to get caught up.
Pretty much the math I did before signing up, for people with a serious case of bookworm the Premium Membership is an absolute money saver (the free PrePubs completely aside)