1/2 cup 2 times a day isn't much for a mastiff at all. Full-grown or puppy.
Neither is 1/2 cup 3 times a day. Growing up, my mom had a st. bernard mix- he could eat all our dogs out of house and home. If I remember correctly, when he was a puppy he at about 2 1/2-3 cups at a time three times a day. Full grown he had 3 cups a day twice a day. Anyway, I'm not sure about big dogs eating puppy food as most of my experience is in smaller breeds. However, my rule of thumb for when I switch my pups off the puppy formula and to the dog is whenever they look like they're not growing anymore and, instead, they're getting fat.