Is PD actually defined as the distance between your "vision-rays" at the surface of your eyeballs, or at the plane of the (manufactured) lenses?

I've been meaning to order glasses online (eg Zenni) forever, but haven't gotten around to it. My theory is that ordering online will allow me to go with several pairs of much cheaper CR-39 and then not care as much if they get scratched, rather than ending up with an expensive proprietary plastic and then wanting to pile on the upsell of scratch resistance, etc. The chromatic aberration of polycarb is horrendous.