The resolution point for a two nozzle machine is usually the centrline and face intercept of the largest nozzle, although it is often not clearly defined. For a four nozzle (compressor) it is usually the shaft centreline, but that's not written in any Code.

A so-called NEMA factor of anything up to 10 (largest I've seen) may be applied to SM-23 allowables when dealing with compressors, but 1.85 or 3 times are the most common. Normally it's 1.0 for steam turbines.

I strongly recommend contacting the machine vendor early in the project to agree both the 'factor' and the resolution point. In addition you should agree the 'Dc' value .... usually it's based on nominal diameter, but could be based on ID's.