You are right in part. You should only decrease the amount of food by 10%. Then once he's lost a bit of weight, go down another 10% until you are feeding for his ideal weight. If you take too much food at a time you are risking depriving your dog of much needed nutrients. There's a technical name for that but I don't remember it. Plus it's hard on your dog's heart to lose too much too soon. Expect 1 lb a month max.