I would suggest a couple of things, first I think you should consider getting a higher quality kibble, Purina puppy chow isnt that great, there is a thread under diet and nutrition that can help you pick one out, but there are several available that are not that expensive but a much better choice. Second, I would look at the feeding suggestions on the bag to get an idea of how much should be fed daily, and third, I would get an accurate measuring scoop.
When I fed 3 times a day, it was within the daily guidlines and how she was doing, i.e. the bag suggested 1.5 cups, she did better on 2 cups, so I fed 2 cups broken into 3 feeding periods. but knowing exactly what your puppy really needs on a daily basis needs to be the starting point.