When I was deciding on a goal weight, I had a VERY rough number in my head, but I knew that number was fluid.
I chose to phase off when I felt I was at a comfortable weight/size (one that I wouldn't have to kill myself to maintain), when I felt ready to start exercising and when I MENTALLY felt like I was up to the challenge of P3 and maintenance.
Could I have gone lower? Yes. I've continued to lose weight in P3 so that tells me I probably could have aimed lower than I did. But I don't want to set myself up for failure and lose so much weight that it's a herculean effort to keep myself at that weight in maintenance. I want to be able to eat, exercise, and be happy with the way I look and feel without stressing about any of it.