Interesting scale,. Number one IMO should be whether or not the animal is interested in eating, so I would tend to put the "hunger" criteria at the top of the scale and give it more weight than for instance pain. This past year I went through a very difficult time with a horse, and I know she endured quite a bit of discomfort…..yeah, pain.....but she never lost her appetite, her eyes were bright, and she was interested in her surroundings, so I supported her as well as I could and she is currently pain free without meds. I didn't feel I should give up on her before she gave up, and in this case I have a good outcome. Some animals are more stoic than others, and will hang on through difficult times. Others give up, and I think they should be allowed a painless end if there is no way to revive their will to live. Age is obviously a factor, as is prognosis. Enduring a difficult time when you know there is light at the end of the tunnel is a lot different from eking out a few weeks or months when the outcome is certain. Knowing whether you are doing it for them or whether you are doing it for you can be difficult to determine.