@ 5' 8" and 128 lb, I would guess with a 40 day fast you'd end up @ 100 lb or less.
My experience has been, 10% of your weight is water and intestine contents and drops off in 1-3 days. Then you would lose 1/2lb a day and your activity level you are likely to lose more in fact. I'd be surprised if you aren't closer to 90 lb.
I have no idea if that's even safe or healthy. I have not been around fasting fora for any length of time to get acquired knowledge for this range of fasting. My calculation are just from experience.
Thanks.
Srinath.