When training companions don't go crazy using up all your training tomes. After you achieve level 10 its good to allow your pirate to level up and obtain higher experience above your crew members. This will allow you to gain 2-3 levels when training your crew with the use of only 1 training tome.
Pirates that only follow the story line without completing side quests will find their game more difficult when it comes to training as about half of the training tomes you receive comes from side quests. Should you find yourself in need of training tomes you can always purchase up to 10 training tomes at the local taverns across the skyway but it'll cost ye an arm and a peg leg.