Don't stress out... remember, they want you to pass. For PADI, study the knowledge reviews, the quick quizes, and know how to use the RDP (recreational dive planner)forwards and backwards. You will need to be able to calculate NDLs (non-descompression limits), for both single, multiple, and triple dives. You will also need to be able to calculate pressure groups and minimum surface intervals after two or three dives. Remeber to draw dive profiles to help you keep everything straight.