It is convention to resolve loads on a two nozzle compressor about the suction, which is usually the larger nozzle. This accords with the NEMA SM-23 code on which API 617 is based, where the steam turbine discharge nozzle is the resolution point, being the larger nozzle. The actual location for resolution along the nozzle axis is also not too clear. The flange face is commonly used.

A four nozzle machne would normally be resolved at the intersection of shaft axis and midpoint between suction nozzles, although this is not to any Code.

It is always best to specify or agree these important details with the machine vendor, as early as possible in the design process.