EPILOG. (Channeling Quinn Martin. If you understand that reference, you’re old like me.)
So strange to wake up this morning with Sweetie gone. My first few tasks were always to let her out, change her water, put out some quickie food (usually a can of sardines or mackerel), get her pills ready, and then go for my walk. Back from the walk (drenched in sweat, as it’s been 78-80 at 6 am here), fix her main breakfast (which she would take all day to eat), give her the pills (each one wrapped in a tiny piece of butter), and then generally keep an eye/ear on her during the rest of my waking hours. And sleeping hours, too, come to that.
This morning I got up, fed my sourdough starter, then walked. No other caretaking tasks. Very strange.
I did get eight hours of sleep last night-- that’s only happened a handful of times in the last few years. My fitbit gave me a very high sleep score of 88-- also never happens. An hour and a half each of deep sleep and REM sleep. One of my girlfriends is convinced I’m sleep-deprived. Very likely true.
I’m glad I didn’t wait a day longer. She was so ready to leave this life and, truth be told, I started to second-guess myself on the way to the animal hospital yesterday morning. But the vet tech who has seen her hundreds of times assured me it was the right thing. The vet reinforced that, which helped.
I went through the Quality of Life questionnaire (cited in the OP) yesterday morning and came up with 28, where it had been in the 40s for the past few months.
I think if you wait until you’re absolutely sure it’s time, you’ve waited too long. At least for me that’s been true in the past.
Missing her, but feeling good and very sure about the decision.
Thanks for all the support as I’ve walked this sad path with my Sweetie, the sweetest dog in the whole world. Y’all are the sweetest Dopers in the whole world. 
P.S. Another thing about the death of pets-- Sweetie joined the household when I was dating the man who died this past February, so she was a link with him. In fact, he named her.
I remember when the last pet died who had been in the household when my late husband was still alive. That was hard.
Both of the two cats I have now knew my dear friend B. who died in June 2018. B (who was a passionate cat lover and was devoted to his two cats) had found one of them as a week-old kitten under a bush near his apartment. No mom, no siblings.
I asked him if he had named her, and he said, “Yes, I’m calling her TLC.”
I said, “What does that stand for?”
He said, “ThelmaLou’s cat.” That’s how she became mine. 
She was very sick for her first few months. Almost died of mange. I used to sing “Me and My Shadow” to her while I bottle-fed her. First cat I ever raised from a tiny kitten. She’s a very healthy 10-year old now and a powerful link to my friend B.
