[Doctor Sax]

Mr Jarrold might correct me here, but I'd be inclined to stop stressing about the length and just write the bloody thing with, say, a minimum commitment of 100k words. Once that first draft is down you can always go back and expand or collapse sections, tinker with stuff, insert new scenes. If it's a work of brilliance but, say, 10,000 words shorter than the market average for your genre or sub-genre, I'd say an agent or editor will advise you of that.