Your PD should always be measured and not estimated. At a minimum the wrong PD can feel like something is wrong with your glasses, but it can also induce eye strain, fatigue, headaches and blurry vision.
If you have a high prescription and the wrong PD these symptoms can often be much worse.
If you are ordering a BiFocal or Progressive lens, even a couple of millimetres off can greatly affect how you see through your lenses
These are two different ways to measure your PD.
A SINGLE PD would be provided by one number and is the measurement of the center of one pupil to the center of the other.