[saghm]

> I have a fool-proof way to do this yourself. Use a mirror and a ruler. Place the ruler on the mirror and stare with your left eye into that same eye's mirror image, such that the ruler's 0 tick is centered on the pupil. Then, open your right eye and close the left. Stare into your right eye across the ruler and note the millimeters. Flip back to the left eye to make sure you haven't moved from zero.

Damn, that's really clever. How did you come up with that?

[kazinator]

It's easy to come up with in the course of experimenting to measure your own inter-pupillary distance.

It goes without saying you will need to look at yourself in a mirror, with a ruler somehow. From the parallax problem is obvious, as is the fact that it's only a problem for mismatched eyes.