I've written a 225,000 word manuscript for a historical novel adapted from a reliably good story, shifted by me into a more modern, popular setting that I love and know a lot about (England during the Wars of Napoleon). The manuscript has been beta read extensively and tightened up and sent back out to betas and gotten encouraging reviews, for what it is worth. But professional editors say its very rare for books now to be of this length anymore. Several people have suggested making it a duology including the betas.
There are not ant books on Amazon about how to books on how to write a duology and this story is not written in the spirit of a series. My grasp of terminology is sketchy, but my understanding is that essentially a writer does not want to have a cliffhanger, or at least the wrong kind. And you need Book One to feel like a completed tale.
Halfway through the story, news of Napoleon Bonaparte's escape from exile reaches England, directly impacting the two military families at the center of the story. So internal and external conflicts among these family and friends get overlaid by news of war and a new conflict.
The aspect that makes me uncertain of how and where to split the book is that the story's main promise and conflict centers on the related themes of honor and redemption, with the protagonist being a riotous, profligate party-animal who, at the halfway mark makes a heartfelt vow to his father that he will turn things around. But of course he does not really fully prove himself until the battle at the end. There are stages and obstacles in which he begins to prove himself, and subplots (friendship and romance arcs, unresolved plot threads that support the main theme). Possible break points are the scene where the news of Napoleon's escape arrives; the scene where the protagonist vow's to his father; the scene where the protagonist puts his old riotous friends on notice he will one day part ways with them; the scene where he succeeds at his schools big Speech Day event just before embarking for the war; or perhaps the scene where the father says his fateful goodbye to his wife (the protagonist's mother) as he leaves for the war.
I certainly can write a new chapter or two into the break point to better create a feeling of a complete story at the end of Book One, but I'm not sure how, and reading of terminology like "unsettled status quo" has me a bit confused, and well, unsettled.