Thank you very much Michael.
I appreciate a lot your answer.

I'll try to make an example
Let's say we know:
- the external diameter D of the pipe
- the design pressure P of the line
- the allowable S
- mill tolerance m
- the internal corrosion allowance
Let's say we have no external corrosion allowance

I understand mill tolerance shall not be considered in pressure thickness calculation, but only in required thickness calculation.
So mill tolerance is not part of ci and co.
This is what I do with "low" pressure wall thickness calculation as per B31.3, and I get the same results of CAESAR II

If I understand correctly, equation 34b shall be used.
I see around some people using equation 34a in the above situation, but I think it is wrong. You can't use equation 34a when you deal with c0=0 and ci ≠ 0.
In this equation we have d = internal diameter
The question is, if we know D and we don't know t, d should be an unknown.
Michael, if I understand your suggestion, I should use equation 35a (or 35c if appropiate).
From this equation I can get the value of T , then calculating d = (D-2T-2ci) and use d in eq 34a.

Before posting, I want to say why I am asking.
I know CAESAR II calculates the tm value. I'm trying to get the same results to check if I am fully aware of how the Code works.

Best regards