It will all depend upon the flavor profile that you are looking for. The flavor from the malted milk powder will be that of a malted milk candy. In work that I did many years ago I used 5% of the flour weight to achieve a characteristic flavor of malted milk, if that's what you're looking for.
5% of 155-grams is 7.75-grams. Using your handy calculator: 155 X 5 (press the "%" key) and read the answer in the display window. Remember, your answer will be in the same weight units that you displayed the flour weight in.
Tom Lehmann/The Dough Doctor