Also traditionally, most authors have a specific word-count in mind, such as 70,000 words, as they write their books.
But when I'm working on a book that I will be publishing, I usually have a specific page-count and price in mind, such as 350 pages/$15.95.
And rather than just spray words onto my monitor, I set up MS Word for the actual page size of my book (such as 6 x 9 inches for a paperback) and correct margins, and start writing a book.
By viewing actual pages, it's much easier to judge my progress, and to know if chapters should be chopped or stretched or shifted.
I alway insert a temporary left-hand "page zero" ahead of the real right-hand "page one" so I can view pages as realistic two-page spreads, instead of onesies, or with left-right-reversals.
This is not very important if a book is all-text, but if you have photos or illustrations or tables, it's important to view the spreads as your readers will see them, to avoid graphic disasters.
Viewing two-page spreads on a modern wide-screen monitor makes it easy to judge when illustrations should be enlarged, reduced or moved around; and to eliminate widows and orphans, and to see if a chapter needs to be trimmed to end at the bottom of a page.