I fed Performatrim, I can't remember exactly it's ingredients if it's a good food or not... Hmmm
But as for how much your dog eats, as you said the bag isn't always whats best for your dog. Remember, exercise has a lot to do with appetite. Try feeding her after an hour long run, or games of fetch.
Roxy barely eats for an 80lb dog. My vet and trainer don't believe me when I tell them, that she eats less than 2 cups a day. She is only really interested in her food if it's mixed with canned. (She does get a lot of treats and table scraps though)
You'll soon find out what's best for your dog. Now, my guys don't get fed a "set" amount everyday. They get their normal breakfast, and depending on how much activity they've had in the day, will determine the size of their dinner.
