To answer your question, a simple way to know is to do a math experiment which I learned in novel school.
1. Start with knowing what GENRE your book will be. (Memoir, Christian, Fiction, etc..)
2. Take 4-6 books (in this case, select E-books) that are the same genre as what yours will be. Try to pick books that are somewhat popular in that genre.
3. Note the number of pages in each book, total them up, then divide the total by the number of books you used.
4. This will be your rough average of the number of pages you should aim for.
Hope this helps...