Correct way to make formula milk?

My formula milk instructions stated 120ml water to 4 scoops of formula - if I measure the water to exactly 120ml before adding the formula powder, I will end up with about 140ml after preparing - is this correct? Most people I know prepare 4 scoops of formula topped with water to 120ml so the entire mixture is still 120ml, to do that it means the water used is less than 120ml. Which way is correct??