In February, Warren Buffett made waves when he explained why a share lull in International Business Machines would benefit his $10.3 billion stock holding. After Wells Fargo and Bank of America passed Tuesday's Federal Reserve stress tests, Buffett's point may also apply to his biggest bank investments. In his annual letter, the investment guru detailed his math on how to gain on stock swoons and share repurchases, using his investment in IBM as an example.