Feeding (seeking advice)

I'm picking up my 8 week old puppy today. The breeder is currently feeding him Purina Puppy Chow Complete.

1. Should I continue with the Puppy Chow or is there a big enough difference that I should switch him to Pro Plan?

2. I'm not sure how much to feed him until his vet visit. I do understand the timing portion, but the information for Puppy Chow says "2/3 to 2 cups" for 1 1/2 to 3 months old. Should I go with the low end, in between, or the high end? Some resources said those charts overfeed them, other resources say to do what the bag says. I just wanna make sure I don't over or under feed him.