Your book's structure is the tool that will gather up your readers and keep them following along with you; it's the tie that binds all the parts of your story together, from end to end, and in the middle too.
Most authors probably wish they had a gauge of some kind to stick into the pages to tell them when their book is done. It’s not just new, inexperienced writers who have that wish. Most published authors I’ve posed the question to say the same thing: it’s hard to know when to put down the virtual pen.
Write with purpose in mind. Edit with purpose in mind. Polish with purpose in mind. Use it as your criterion for chopping (or lack of it) and gauge your satisfaction against it. When 100% of your words are charged with meaning, your book is done.