One of the rules of thumb I've heard is: in lieu of allowable nozzle loads, if the pipe stress at the nozzle is less than 5,000 psi (or 10% allowable), then the nozzle loads are probably acceptable.

I would assume this stress is found in the operating case reports where the forces are "real". Whereas the occasional/final load cases used for evaluating the allowable stresses contain "fake" forces due to the matrix math involved.

Is this correct, and are there any other guidelines/references to similar rules of thumbs?