word count
I always prefer to work based on an estimate of words per page. It's so much easier than fiddling about trying to get a "proper" count out of a word processor.
The way I did it: take a sample of your writing, a few thousand words, and copy and paste it into a new document. Format it as you would a normal manuscript (e.g., Courier New, 11pt, double space, 3cm margins, 3cm first-line indent on A4 for me). Then join everything together into a single paragraph. Once you've done that, delete everything on the last page. Now use the word processor's count to get the actual number of words (or use the character count and divide by 6 as suggested above, if you feel this is better). Divide by the number of full pages you have, and that's your number of words per page. For me, it's 275. From this point on, work in terms of complete pages.
The advantage to this approach is it takes into account the volume of whitespace in your manuscript, as well as the number of words, which helps estimate the eventual length of your book. The number of pages in the book is the important thing, to everyone concerned, but to work it out, you'll need to know both your number of words per page and the same figure for a book printed in the style yours will be. I've looked at a few standard paperback books, and most have between 300 and 400 words per page, so will have slightly fewer pages than you have manuscript pages (assuming your format is similar to mine; if you use Times New Roman as your font, for example, you may get substantially more words per page than me)