your hunger im guessing probably only needs to be under 16% but everytime I've done it I've been at 0% I guess, I get regurgitated by her and while she's reeling from it you have an opportunity to eat her, unless it was stealth nerfed in the last patch. On normal mode its also possible to eat her while she has a normal sized student in her if you're hungry....you can't even try eating her if she's pursuing anyone though.