From my understanding, calculations are weighted a touch more than the other questions. So, in my opinion, I would work through the APhA problems in Chapter 1 until they are planted firmly in your head. These are almost gimme answers if you learn how to do them. Granted, there may be instances where you will have to guess. But, by and large, I believe if you prepare you will see the results on your score.
P.S. Don't forget about the nutrition calcs!!