Every dog is different and that is the major reason that the bag guidelines don't always work. It is important not to go to far *below* the guidelines or you are at risk for being deficient in nutrients for your pup.
I recommend evaluating the Purina BCS chart and then comparing your dog to that. A 5 is ideal with an easily defined waste and ribs not visible but easily felt when petting him. For the large dogs I tend to recommend staying on the light side of normal. Every pound adds 4lbs of force on the joints and since that is a major issue with big dogs every pound can make a huge difference. Even as a puppy he can be evaluated with this scale effectively.
Not seeing your dog and knowing how much you feed it is hard to say whether you are feeding appropriately for your pup. I hope these guidelines help.
here is the link to the BCS chart.