Repeat for precision a few times and use an average for your ultimate metric. Stand before a mirror or ask another person to measure for you.
Give the sign visible with the magnetic strip to the middle of your chin just below your lips. If you can not obtain or would rather do so yourself from an optician, here are a number of ways to obtain this measurement. Here are some ways to measure this yourself if they don’t give you it.
You can measure your PD for your glasses in many ways. Your PD is in millimeters from the center of your eyes.
Printable Pd Ruler