How to give the cat food, so he lost weight?

In cat weight( about 8 kg), the vet said to put him on a diet.Our pet usually eats dry food Royal Canin reduced calorie, genuine food consumes only a little bit of finely chopped raw beef.Kitten Scottish Fold, recently neutered, age 4 years with little.

Question: for how many grams of dry food to give the cat to lose weight and be sure whether in this case to cancel the raw meat?