You are right, of course.
Yes, in some cases we need to look at that piping flange with more accurate methods.
And which method is practical for the piping flanges?
I expect the next flange calculator will be exactly this tool. That means a tool that not just guesses the gasket load but calculates numerically it. If this numerical result shall be in range of 10-15% of the theory result, that means also the theory is good. If the difference is 50% it’s a complete other discussion.

NC-3658.3 says: The pressure shall not exceed "X" times the rated pressure. What X do you consider for a piping flange which is not under NC-3658.3 requirements?

Best regards