10 Grams max in a 5 gallon batch is the most you should add. Any more will strip color and hop flavor and aroma. This is all from George Fix in Principals of Brewing Technology.
From my own experience i use a level 1/2 tea spoon with about 4 oz boiled cooled water. Make sure the Beer is cold a possible. I dosed a room temp beer and it didn't turn our as well.