How dp they determine how much protein you should eat/

Options
I've been on Nutrisystem for about 2 months and have lost 13 pounds, but can't afford it anymore. I found this site on line and thought it was a perfect way to stay on track, but I went over by 18 on my protein and I don't understand why they are limiting my proteins. I didn't go over on my calories and was pretty good on the rest of my limitations. I thought high protein was a good thing.