Fluid passing through a convergent-divergent nozzle

[T C]

Now, you likely will have separation and therefore a different effective area ratio than the actual physical nozzle, but the fundamental principles remain the same.

How to determine the effective area for such pressure level?

 

[boneh3ad]

How to determine the effective area for such pressure level?

Pressure has little to do with it, and at any rate, you aren't going to find a simple way to calculate the effects of separation accurately without resorting to CFD.