If I write in "MS Word" single space, how many pages would I need for a 200 page book? It's not one page in Word= One page of a book is it? I wouldn't think so.
How many characters are in a representative page? i.e. solid text, no headings? Check File, Properties, and Statistics; the character count without spaces is the number you need.
Sorry for the multiple posts; not firing all cylinders right now. Take that number for the rep page and divide it by 2400, and the number will tell you how much shy or how much over a standard book page it will be. I standard rule is to set 40 lines per page no longer than 60 characters in length.
Can you give an example of a book the way you want it to look though? Some books have more quantity per page than others. Holiness by Ryle for example has more text per page than God Loves You by Spurgeon. It would take about 2 Spurgeon pages to fill 1 MS Word page. etc
Well, yes, design often is governed by content. A Children's book will have very little text per page compared to a textbook; a devotional book may be somewhere in between.
You should like the design, but you should give greater weight to the intended reader. To use an extreme for instance, you would not set Calvin's Institutes the same way you would Fun with Dick and Jane.