On page 2-356 of Perry 7th edn, reference is made to the API Technical Data Book for Pr > 1.0, which relies on the Pitzer corresponding states model.
But first, you'll have to determine if this ampoule will indeed go into dense phase ( Pr >1.0). The NIST link should help with calculating Zc for this - since this is a student posting (?), you'll have to figure this out yourself.