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.If your dog needs to lose a few pounds, there's another very highly rated food that has worked for an obese dog I've fostered in the past. Have you tried Wellness Core Low Fat? It works great!
Just remember, no matter what food you feed, feed the amount needed for the desired weight, not the current weight. Both Acana and NVI are good foods; I've used both.