That's hard. Like Veronica said, just try to be in the moment and not torture yourself with thinking and guessing. At least you know what's coming and have time to prepare. In some ways, the desperate hoping and the not knowing whether they'll recover or not can be worse than knowing for sure that their time is approaching.

When Maggie is really ready, I think it will be more obvious. As long as she seems happy and eats a little, I think maybe it's not time yet. She might have a day without eating which could be just a normal fluctuation in her weakened condition, or a morning when it's hard for her to get up for a while. When they are really failing for the last time, it's more obvious. I'm guessing from your posts that you only want to prevent the last phase when they are typically very uncomfortable and completely weak.
Your strength and your love will pull you through.