Bob,
This is really a question for the pump vendor.....or sales rep
The required thickness of a pump flange is dependent on the casing material used as well as the maximum operating temperature used in the design of the pump.
Pump vendors typically design a flange to match the dimensional requirements of the adjoining piping (probably ANSI Class 150 or 300 in your case).....then they "add a margin" to the flange thickness and hydrostatically test to 1.5 times the deign pressure.
I know that this is a long answer.......but there is no calculation proceedure for such a situation
If this is an old pump, I would not hesitate to ask a competitive pump vendor this question.
Good luck....
MJC