I would suggest that that is only partially true, and not "generally" true. Some brands make multiple varieties of the same quality of food, but they still have different protein/fat contents. Additionally, I would think the source of protein could be more critical than the amount of it. I have heard some proteins used in some feedsare harder to digest.

I would be more inclined to say that, if there was a #1 rule for dog food, that it would be ensuring that the primary ingredient is a quality ingredient, like lamb, chicken or beef. I will take lamb and rice as my top dog food ingredient over corn any day, and I will look at that before I look at protein/fat content. As far as protein/fat content, I like it to be about 26/16 or maybe 26/18 with a real meat for a primary ingredient. I have super lean dogs that seem to do real poorly on high energy sporting dog feed such as 30/20.

My next interest would be ensuring the food contains other healthy substances such as fatty acids.

That said, this is a good topic and a good opinion of yours to kick off the discussion. My opinion doesn't make me right, just different.