Whenever having one of these hospice type vet appointments, I always ask this:
What, specifically, do I need to watch for that indicates the downturn? And by that, I mean what signals the point of no return, where things are just going to get worse?
I always want to make the call at that point, at the latest. Sometimes they have brief rallies, and it sure is hard. But when you know things are just going to get worse, it’s such a kindness to let them go when things are still good, before they get bad, before the animal wants to die. Missing that point just sucks so much 
I really applaud you for having this conversation with the vet and thinking ahead. It’s so hard. I’m so sorry.