I'm writing a nonfiction with a focus on history and have one chapter lacking in length. Of course I am working on expanding it, but I fear it might not be quite enough to fix the gap (currently it's a 40-page gap).

Through my research work in college and article writing, I noticed that sacred attention is given to the symmetry of parts and sections. Also most nonfiction material I've read has been quite consistent in distributing the book over chapters more or less equal in size.

I'm aware that I don't need a perfectly equal division, but how much discrepancy is tolerable?