[boptom]

Progressive lenses need much more accuracy to be fitted correctly. This means a mono-PD (separate PD for each eye, not just the distance PD halved) and also a height, usually from "datum" measured for each eye too. This measurement would be different depending on the frame and how it sits/adjusted to your face.

This means the frame must be chosen and fitted to you before any of these are measured.

It becomes a 2D measurement.

There is a lot more to it (vertex distance, pantoscopic tilt etc) but the above is the absolute minimum.