I agree with jte and others. To add my own info: It's been my experience that the neck thickness of these forgings may vary with the manufacturer (since there is no "standard" that addresses these forgings). So be wary of any calculations based on the LWN.
ASME B&PV Code Section VIII Div 1 added new paragraph UCS-66(c)(4) a couple years ago to address equivalence of LWN flanges to standard flanges when considering MDMT, there are some restrictions placed on the dimensions so be certain that the LWN forging meets this rule if designing to Section VIII.