The bag should tell you how much to feed, its different for every brand of food. Each pup varies too as to how much they need which is why any guide is just that. Ultimately the only way to know if they are having too little, just right, or too much is by looking at them. If they are too fat cut back, too thin feed more and just right stay as you are.
What I would urge you to do is look at changing to a complete food that is natural. Unfortunately Pedigree is full of alot of rubbish. Try looking at Burns, James Wellbeloved, Arden Grange which are dry complete foods. There are other natural dry foods too which are escaping me at the moment but I'm sure someone will post the names shortly.