Reason for stopping: Too tired to see straight. I am in fact stopping in the middle of a sentence, because I more or less know what happens next.
I am very pleased that I not only figured out what Will is up to today (or, more precisely, Kit figured it out, because Will has very cleverly managed to hide his agenda from me. He only lives inside my head. Nonwriters, don't worry: this is in fact a normal part of the creative process.) but I also managed to nail down the motivation for a plot twist at the end of Act 2. So, probably in about 150 pages. :-P
Heh. It's all good. Now I'd better go to bed, because the dogs will likely wake me up at 5 am.