My optometrist refused to give it to me unless I bought glasses from them. They claimed the number depends on the specific glasses you have.
Fortunately I was able to find it on a prescription from a previous provider.
Fortunately I was able to find it on a prescription from a previous provider.
However, where the optical centre of a lens is placed within a frame can vary from the PD a fair bit, depending on how the frames sit on your face, as the contact point of the frames do not always line up perfectly with the top of the nose bridge.
Also, an oft looked over measurement is the distance from the bottom of the frame to the centre of the lens, which is important with complex prescriptions and cannot be measured without looking at the frame on your face.
Having said this, with most low prescriptions the human tolerance is so high that it does not matter.
I guess I was lucky to some extent, and I hope we're all able to find "better" providers.