Instead of being a stupid question, I consider this a can of worms question. And here I am, with the first wiggly, juicy worm.
Usually, what works predictively to tell how much space you would be taking up on the printed page within a book is not comparing a screen page or a word processor output page to a book-sized page, but using word count. This is why editors ask for word counts on manuscripts.
In printer speak, as I understand it, word count is not truly a count of words but a character count (six characters per word! Spaces count. As do punctuations marks). Given that, your word processor's word count function is not a true word count from a publisher's point of view.
The most illuminating, concise version of how to get an accurate word count I've ever read comes from
here. All hail Teresa.
However, I just use wc -w and consider that close enough.
Wait, is that my geekitude showing?