Yani,

When you use Fig. G in ASME II D with a L/Do ratio > 50 then you use L/Do = 50.

How has the external pressure been specified? If your component is single skin (no jacket) then the maximum external pressure that can be achieved is when the inside is fully evacuated. In that instance your external pressure will be, as you have said, atmospheric pressure (1 bar extenal).

You may find more info if you search in the Pressure Vessels Forum instead. If one of your colleagues uses PVElite then they will probably be able to do the calculation for you very quickly.