Yani,

That sounds like you have the correct external pressure for calculation. Can I just add one other point. Designing for external pressure using ASME VIII Div 1 (by comparison to other codes) can result in a small margin of safety in avoiding collapse. UG-28 (f) (see footnote) warns about designing for partial vacuum conditions and suggests that an additional margin should be used so you do not come close to the collapse pressure. Some codes recommend designing for full vacuum even if you have a partial vacuum condition. If you are designing for a partial vacuum condition you need to be certain that 0.21 barg external is a maximum or else you must use a higher value.