A dispersion model (as used in emission calculations)could be used to calculate concentration levels under varying circumstances to determine "safe" distances or heights. Explosion and exposure (& sometimes odor) limits should be incorporated into the model to arrive at the "safe" exhaust position.
The U value you're using is extremely high. Have you taken any tank insulation into account in your U value determination?
For an insulated tank I would expect a value that is a factor of 25 to 100 lower.
