The maximum allowable expansion case stress can be calculated using the conservative method, where the maximum allowable is calculated and used. A second option (the one CAESAR II uses by default) is the progressive formula, where the stress check is done using the following method:
max allowable progressive exp stress = max all. exp stress + max all. sus stress - occuring sus stress.
This formula is also given in the EN-13480-3 (formula for Sigma4) (and other piping codes by the way).