Assuming that you are referring to ASME Section VIII, Division 2 - the allowable stresses are obtained from ASME Section II, Part D, Tables 5A and 5B. One does not "calculate" an allowable stress - one obtains it from the Table(s).
ASME Section II, Part D, Appendix 10 details the calculations used to determine the allowable stresses in Table 5A and 5B.
Nevertheless, the minimum-specified values for yield and ultimate are used. It is NOT permitted to re-calculate the allowable stresses upon receipt of the material MTRs with actual values for yield and ultimate.