I once tried to measure my own pupillary distance using the ruler-mirror method.

I failed and the glasses I ordered were unusable. In the end I brought them in to my optician's office where they precisely measured not only my PD but a couple of other metrics that I think described the frame/lens position on my face.

I sent all those measurements to the vendor (https://www.warbyparker.com) and the second glasses were perfect.

These other self-measure methods look better than the ruler-mirror method that I did, but if I ever buy glasses online again, I'm going to take the trial frames straight to the optician's for proper measurements.