I second MoverZ opinion: cherry picking equations and applying out of context is always dangerous.

In this particular case, that formula was first included in a work which was not in connection with Nuclear Code; it is Rodabaugh and Moore's "EVALUATION OF THE BOLTING AND FLANGES OF ANSI B16.5 FLANGED JOINTS".
The formula appears to be "dangerous" simple; in fact it is rather a simplistic "envelope" following complex stress flange calculations developed by authors. Later the formula was proposed by Mr. Rodabaugh for Nuclear Code.

For this reason, I think it is not "out of context" to consider the formula as a "reasonably good approximation of the moment limits calculated" for ANSI B16.5 flange.
Remains the reference problem; you would refer to that article not to ASME III!