How many people adjust their font size to match the number of lines seen on a book page? Not so important on an ebook, obviously, but do you do so just in case your work reaches mass production? If anything it at least gives a good idea of the actual page number my work has reached.
Your word count should give you a good idea of the page length shouldn't it? I'm assuming you mean a paperback or hardback sized page? I think dividing the word count by 300 should be a good estimate. Of course, this will vary slightly depending on the amount of whitespace in the work. When I'm using Scrivener (which already has a page estimator in addition to the current word count and session progress) I do adjust the font/margins to paperback, but it's mainly so when I view the pdf output I can get a better visual on the amount of whitespace on a single page, the pattern/rhythm in paragraph sizes, etc. (I tend to have too many paragraph breaks).
I'm probably in a small minority (possibly a minority of one), but before I get very far into a manuscript I set up the book's format in Word and write in the actual format. But I do that because I can't help it -- my education and background was graphic arts before I got into technical writing, so my brain tends to freeze up when I can't see what my work product is going to look like. Once I have the page size and margins established, then I have to decide what I'm going to use for the typeface, and then the type size. It all matters; Times New Roman is a lot more compact, horizontally, than Bookman Old Style or Century Schoolbook even in the same type size. The late typography guru, Kathlees Tinkel, suggests that a line of text should contain between 60 and 70 characters, and the type should be between 9-point and 12-point. https://www.uwosh.edu/faculty_staff/jager/typetinkel By working this way, I not only always know what my pages will look like, I also know exactly how many pages I have. I do understand, though, that many people would find my approach to be paralyzing rather than freeing.
I always set the spacing, font, and font sizes for every story I write in Microsoft Word. It's just so damn satisfying to make my stories look as "official" as possible, picking different fonts for chapter headers and so forth. Most of my stories are typed with Times New Roman, but Book Antiqua looks perfect for my current fantasy story. But this then means that it might be best for me to judge the lengths of my stories by their word count, not the page count, since the words per page will be different from story to story based on how I format it.
Yes, word count can be more important than page count because most editors have specific word-count requirements for each genre of their publication. It's been my experience that editors also have specific preferences for font and font size. I have a young adult fantasy, for example, that will never see a wider audience than family and friends because it doesn't meet the word-count requirements of editors nor the length expectations of readers who like that genre. Maybe someday I'll be willing to solve the quality (concise, evocative prose) versus quantity (word count) dilemma. My wife, however, is quick to remind me that there are seven days in a week and Someday is not one of them...
And that's an important point that I overlooked in my previous reply. For submissions to agents and traditional publishers, word count is the important factor and the manuscript should be formatted according to the publisher's or agent's requirements. My comments about preferring to work on a page formatted to the size of the book's trim size was based on self-publishing, where the number of pages is an important criterion as well as word count. In traditional publishing, the publisher will [probably] design the book interior and take care of formatting and typography, so there's no point in worrying about typeface or layout or type size. This is a decent exposition of a generally-accepted manuscript format: https://www.masterclass.com/articles/how-to-format-a-book-manuscript#how-to-format-your-book-manuscript But the OP asked about lines length: I guess that's why I jumped to the assumption that the question was about self-publishing, because once you get into commercial publishing there is no standard "number of lines seen on a book page." That said, since most printed books are roughly half the size of a sheet of 8-1/2 x 11 typing paper, to adjust the font size of a manuscript to approximate the line length and line count of a hypothetical printed book would make the type size so big it would be like reading a book for 1st graders.
Yes, the fluid nature of e-books means authors don't have to adjust font and font size in their source document. Authors should, however, concern themselves with two things that can affect line length in an e-book. 1) Hyphens and dashes; and 2) Long lines in poetry. Below are links to articles I just posted to help authors deal with each; pending their approval by an Administrator, of course. https://www.writingforums.org/threads/hyphens-and-dashes.171746/ https://www.writingforums.org/threads/formatting-poetry-for-long-lines.171747/
