Math Problems at IMF?

IMF published their new Financial Stability Report, in which they claim that U.S. banks will have $250B drain on equity due to writeoffs in the next two years. The simple math they go through is follows: total write-offs on loans and securities over the cycle for U.S. banks will be $1050B. $500B of that has already been recognized, which leaves $550B remaining. They estimate after-tax pre-provision earnings in 2009-2010 at U.S. banks will be $300B. $550-300B=250B drag on capital. But what happened to applying a tax rate to write-downs, which would presumably reduce net write-down number to 360B, or result in only $60B drag.

This not so well-thought out math only adds fuel to the fire burning the bank stocks and shaking the confidence in the system. Lets hope Fed's stress test is more well-thought out, and somebody actually pays attention, rather than just saying 'they are all bankrupt, I don't care what numbers say', like many people seem to be prone to do these days.