1%

1% doesn't sound like a lot, but think of it as 2 pages of a 200 page book. Any day you can write that, or even half of that, calls for celebration. 

Think that scholars write 1-3 books in their careers and are fine with that. Say the tenure book, the full professor book, and one after that. Some write more, some stop at 2.  

So to get one of these books done, you need 200 days of rather productive writing, which means writing a page. This should be doable. 

So this means the trick is not writing 1% or .5% of your book on any particular day.  That is easy enough. What has to happen is have enough days like that so that you reach 100.