Do not take Fire D for granted. Practice it until you are blue in the face. Get use to both the Observer's and Gun sequence. The more comfortable you are with that the more natural everyting else will come. You will be expected to take orders from the OP, calculate data, supervise your crew and give orders to the guns, all concurrently. A good CP crew is smooth and efficient and without any panic. Knowing the basics cold with assist you greatly. Too many guys panic on simple crap because they are weak or mediocre at Fire D.
For calculating, practice basic arithmetic until you can do it in your sleep and get use to picking grids on a map quickly. Familiarize yourself with a set of TFTs and practice interpolation for extracting elevations and fuse settings.
For IFCCS, don't sweat it. If you already have the basics down, filling in a computer page with information that is given to you is very easy.