Certainly not less than the MAWP of the attached pipe. For the latter you could safely consider the flange ring as a line of support (you can check this condition, but guess is satisfied with a large margin).
Or are you in a special situation?
Refer to ASME Section VIII, Div. 1, 2004 Edition, Appendix 2, Paragraph 2-11. You could iterate the formulas backwards until the limiting maximum allowable code stress is obtained to determine the MAWP for external pressure conditions.
Also, remember that these calculations are for stress compliance checks only and do not necessarily assure leak tightness. Appropriate means, using analytical evaluations coupled with experience shall be used to assure leak tightness at the calculated external MAWP.
