There is a common misconception surrounding karma. It really has nothing to do with a karmic bank of good and bad deeds. This is because any deed you do can have a wide range of effects. If you looked at that with the outdated version of karma, it would leave you very confused wouldn't it?
The idea is that you have chosen your experience; you don't have to experience what makes you feel uncomfortable or shut down, you can choose to send a situation into a new direction at any time. The concept of Karma will challenge a person's way of looking at the world previous to incorporating it into their lives. A time when you feel like life is not fair, when you fantasize about a knight in shining armor or a sudden windfall of prosperity and happiness. Karma is meant to teach that life is not about luck, and that you are not powerless in the face of upheavals. Every experience has something teach you, you are to learn how to create experiences that make you feel fulfilled. In other words, you can achieve what ever type of happiness you can imagine. The law of Karma as well as all other spiritual laws come into a person's life at the right time. They are meant to get you back on track to a life of purpose and happiness.
The karmic bank idea is essentially useless, and I'm not sure how karma could be confused. For example, you help an old lady carry her groceries across the street. Don't expect to experience some sort of reward in the future. It would be foolish to add up deeds like this into a bank inside your head. You are simply wasting your time and are selfishly motivated. The karmic effect in this situation is the inner happiness you feel.
You can judge whether or not you are on the right path by assessing your happiness and behavioral patterns. If karma is going to be understood and used in a persons life than it needs to be broken down into something you can understand and use throughout your day. It needs to be applied to the moment that you're in. Otherwise how can it be grasped?