That definitely helps - the reason I asked the question is that my club boiler inspector passed my plans for a 6 inch diameter vertical boiler at 100 psi using 3mm thick copper plate. This was in 2018 (you can view the build in and it was certificated against AMBSC code Part 1 Issue 5 1984...
Whats your opinion about the operational difference between 3mm copper sheet and the 1/8th copper sheet requirement for a NWP of 100 psi. Is this the de-rating you were referring to?