Hi and welcome to COL.
Pictures of George are a must!
It's hard to advise on the food, I have had a quick look and the 3 manufactures food do vary on moisture, protein and fat content so that will probably count for the recommended difference in feeding, you are probably best to feed accordingly to each manufacturers recommendation.
With Reese now he is older I have learnt by the way he looks if his food needs adjusting, but as a puppy it was impossible! I could look at him one day and think he was getting a bit chubby then overnight almost he would have a growth spurt and look too skinny! I don't think it's really anything someone could tell you exactly, it's more knowing your dog, his energy level and finding the right level for him.
I did take him in regularly to the vets when he was young, to help get him used to going but also the nurse would weigh him and advise me on his body condition. I could have easily over fed him without this support as I was used to big chunky Clumbers!