You can calculate the junction temp based on the Theta_JA parameter and the expected PLoss.
As an example For EN6347QA: Theta_JA=30C/W (Please check the datasheet page 5)
I hope that example is clear on how to calculate the expected junction temp.
Yes I know how to calculate the junction from Theta_JA. However, this is not very helpful as I need to measurethe junction when the chip is in vacuum and here the Theta_JA is not any good. I need to know the junction to top case so I can measure the top case temperature and the calculate the junction