As far as suggested feeding on the back of the bag goes: the companies usually suggest more food than a dog needs.
An important note to add to dog food labeling is that the quality of contents can be determined by the order and verbiage used to describe the contents. For example, "beef, fish, lamb..." means 95% meat by weight, where "beef for dog" means 70%+ meat, "beef dinner, entree, platter, etc" only means 25%+ meat, "dog food with beef" only must have 3% meat, and "beef-flavored" only needs to taste like beef, but contain no meat.
The same creative marketing skills are employed in the grain content. That is why some dog food looks like it doesn't have much grain in it when it does. Instead of just listing corn, soybean, wheat, wahtever, the company breaks it down into smaller catagories to throw the consumer off balance like this: flour, meal, bran and middlings. That way they can list them down the list of ingredients, but if added all together then the grain in many brands would be the first item listed. I'm sure I have described this as clear as mud!
Spending time researching your brands and contents of each bag is important and will be seen in smaller bowel movements resulting in higher nutritional absorption by your dog.