Select to take the intersections of the cone and the two cylinders as lines of support for external pressure, provided that the moment of inertia and area of reinforcement requirements of ASME Appendix 1-8 are satisfied. Alternately, you can calculate external pressure using an equivalent design length which includes the cone and both the large and small cylinders. For details see Section VIII, Division 1, Paragraph UG-28 and Figure UG-28.1 (A-90 Addenda and following.)
Normally it is preferable to take the cone as lines of support, because the equivalent length of the large cylinder/cone/small cylinder combination may easily result in low allowable external pressures. However, the moment of inertia is very easy to be less than the required for knuckle-to-cylinder junction — because the shell/knuckle/cone is usually so close to the resulting neutral axis. The moment of inertia with the knuckle is calculated, following the procedure of code example L-3.3.