I would study until you feel comforable. Obviously, you have doubts, and you want ot be confident before taking the test.
Here is my suggetion. Do as many practice probs as you can get your hands on. Studying the material isn't enough, you must work practice problems until your fingers and eyes hurt. Work problems, study, work problems, study....... Then, buy the Topscore tests and see how you do. If you score 17+ on Topscore, you should feel confident going into the real thing.