Enter the actual metal temperature for the shell at the Tubesheet, under a realistic operating condition.
It is important, especially when evaluating fixed tubesheets without expansion joints, that you enter accurate values for metal temperatures for each operating condition. You may have to run the analysis more than once to check several metal temperature cases. Frequently the metal temperatures will be less severe than the design temperatures, due to thermal resistances. For example, if the shellside fluid has a good heat transfer coefficient and the tubeside fluid has a relatively poor heat transfer coefficient, then the tube temperature will be quite close to the shell temperature.
Don't forget to evaluate the condition of shellside or tubeside loss of fluid. Especially for shellside loss of fluid, this design condition may govern the exchanger design.