You're asking the wrong question, and potentially using the wrong analytical tool.
First, do you know, for certain, that your structure will remain elastic at the critical buckling load? Or, more specifically, will your average/through-thickness stress be less than 55% of yield at the critical buckling load. If so, then carry on with your eigenvalue buckling analysis. If not, then you will find no answers in an eigenvalue buckling analysis - and you will need to evaluate an elastic-plastic buckling solution, including initial imperfections (you have considered fabrication tolerances in your initial imperfections, right? Because there is an interplay between the magnitude of imperfections and design margin (note that I absolutely LOATHE the term "factor of safety" or the even more dubious "margin of safety")).