pipe thikness calculation

Your question is not clear. Is it possible for you to give more information.
Example:
What is your role? (Are you the Client?, the Project Manager?, a Piping Designer?, A Process Engineer? or what?)
Why are you asking this question?
If you are not the Piping Material Engineer who developed the Specs, have you asked him/her this question?

I think that you are mixing up two different things.

The Flange Class is based on the pressure and temperature ratings for the flange.

The wall thickness of the pipe is based on the line pressure, line temperature, the corrosive effect of the commodity and the expected design life of the plant.

Based on this the wall thickness of the pipe used with your class 150 and class 300 flanges was not selected based on the flange class.