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.
The emotional map of your book is different than the plot, though the two are tightly related. Being aware of this emotional current can bring clarity to your writing, and is a powerful way to progress your story.
A micro-memoir focuses on one moment – maybe only seconds long. Whether it's the actual start to your book, or just a way to get you to start the writing process, these little scenes will set the stage for the larger story.