It’s because it’s part of the ‘fitting’ of the glasses rather than the prescription. Or at least that’s what they’ll argue. The thing is it affects the manufacture of the lens so in my opinion it should be part of the prescription.Yea, my optician didn't give that on the print out of my latest eye test. I'll contact them and see but suspect they won't but doesn't look too challenging to measure with some help.
I winged it once, tried to measure my PD manually and bought online. I got by with those glasses, but they were never ‘right’.