Coffee and Horrible Manners

George, a waiter at a restaurant, was having a little bit of a difficult time with his customer. His customer was yelling, "I want exactly 200 mL of coffee in my cup! Then add the cream! If the amount of cream is not exactly 20% of the coffee in the cup, I'll have you fired! I have friends in high places you know!"

George smoothly pulled out a measuring cup and measured exactly 200 mL of coffee [he was an experienced waiter when it came to these things] and was about to add in 40 mL of cream [20% of 200=40] when he realized that this customer was trying to trick him. If he poured in the 40 mL, the amount of liquid in the cup would be 240 mL, and there wouldn't be 20% cream anymore.

So then, how much cream should George put in to make sure his customer [who is suffering from poor temper management] is happy?