If my memory serves me correctly, I once a used a digital try-on app from an online glasses retailer and they had me hold a common item with a known width (something like a credit card or drivers license) in front of the cam within a bounding box. It could guess the location of my pupils but allowed me to drag the pupil location markers to the exact centers. Calculating pupillary distance was then just a matter of using the distance in pixels between the pupils and converting them to mm using the ratio of pixels to mm of the card in the previous photo.
