The feeding amounts on the packet are only a guide,some dogs need more,others less. I would go with your dogs body condition,too fat,cut down,too thin,up the amount. You could try feeding your dog more and see how he does :) When you run your hand down his back can you just feel his spine and ribs without digging around,this would indicate he s an ok weight. If you have to feel around for his ribs as you cant seem to find them,then he is too fat.If you can easily see and feel his ribs,he is too thin. Does your dog fit into any of the above?
As for telling you how much he needs,i would not have a clue