Each vendor and each application has different rules or formulas. For Thermal Power plants that use boilers and steam turbines one may work backwards from the steam turbine stop valve inlet pressure during 5% overpressure operation plus any margin needed to allow the operation of a power actuated relief valve plus the presure drop in the steam transfer pipe plus any margin cuased by the machining tolerance in the steam turbine blade manufacturing process. In the old days, before numerically controlled lathes, that last margin could be as high as 5%. During a steam turbine trip , the boiler's pressure safety valves will lift and cause an overpressure above their lifting pressure by as much as 3% , so the design pressure may need to be set for meeting that condition.
The design temperature for transfer piping ( not heat transfer tubing) should be set with sufficient margin above the average predicted maximum operating temperature to account for errors in prediction of upstream heat transfer plus errors in the process temperature control and monitoring components. Over time, the oxidation of the temperature monitoring thermocouple junction can cause an error as high as 5F.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick