when pap is overcome with grief, that should be the half way point, like when sans tries to spare you, and then after that he flips out with his orange eye and the text should say "papyrus no longer believes in you"
Quite honestly I don't like this because it feels like The Great Papyrus just gives up, where as I would like to think he would be determined to save the human. I think after he gets hit, he would be determined to hang on and would go to the AU Disbelief paps.