Maybe this brands me as a little too geeky, but what I do is find a couple of books that I respect enough to emulate length-wise, then scan a typical page, OCR it, import it into Word, count the words on that page, then multiply by the number of pages in the novel. I generally reduce the final word count by 5% to compensate for chapter-end pages that don't take a full sheet. This will give you a pretty accurate word count, and it's certainly better than guessing.

On the other hand, I was shooting for 100,000 words on The Cunning Blood, and ended up with 144,000 words, because that's what it took to tie up all the loose ends. You are not always (completely) the boss of your work.