[tremendo]

I have done this a few times, and got inconsistent results. Distance to mirror, or camera to face, positioning of the ruler, they all seem to induce variability. Once I got a reproducible result and went with that, ordered progressive glasses online and they were… off. On another occasion I got PD (and prescription) numbers from optometrist at Visionworks, then took numbers and ordered from Zenni, and they were… off again. Also this PD number again different from the one I had measured myself. But when ordering directly at the store they were perfect. I even wondered whether they actually gave me the wrong PD number, to "teach" me a lesson not to go order elsewhere. Sigh.

[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.