When designing converters, the average junction temperature of the semiconductor is a frequently required estimate. Its analytical calculation requires the total thermal resistance of the cooling arrangement. Unfortunately, due to the complexity of the heat dissipation processes, an estimate of the thermal resistance is usually associated with low accuracy. To significantly improve the accuracy of the simple truncated cone model, effective heat spreading angles are derived and applied in this work, a published concept that allows the replication of the thermal resistance of an FE simulation. Simplicity and accuracy of the resulting model are demonstrated on the natural convection cooling design of a discrete surface-mount IGBT in a TO-263 package. The validation of the modeled total thermal resistance yields a relative error of 4% with respect to the measurement result.