I don’t think once implemented should be too hard to be able to split a book in a custom way, back to the original suggestion of having a custom calendar.
To begin with it has to be able to accommodate to some leeway, as there are books with slight differences in page lengths, and sometimes even chapters.
A quick workaround could also be made until it’s fully implemented the custom splitting, by creating threads/splits like chapter 1 1.5 2 2.5 etc.
