If you are an engineer trainee then you should be giving this question to your supervisor, but in answer to your question I would say that you cannot really calculate for this situation. Rather, you should be referring to standards for pressure fittings, and then pressure testing (hydrostatic pressure testing) of the actual fittings. Test pressures are obviously a significant magnitude higher that working pressures.
In the case of machining one-off fittings, for use in non critical applications and for lower rather than higher working pressures, then generally a person would copy an existing fitting (standards compliant, pressure rated) in terms of dimension and material. High pressure applications should be considered very carefully...