how to obtain your accurate PD value?

[glasses lens]

my script has a comment: Dist P.D. r 28.5 and l 32.5, at first, I can't understand this information. BUt, their customer service tells me that the accurate PD measurement is necessary to ensure the correct positioning of your lenses within the frame you have chosen. There is a guideline, that is most *****s PD's are between 55-65mm (2.2-2.8) inches and most kids PD's are between 42-54mm (1.6-2.1 inches). The optical center of eyeglass lenses is the part that gives you the truest vision, and it should be directly in front of your pupils of eyes.