I just asked my optometrist for the inter-pupillary distance and he added it to my prescription. Also, when I got lenses in existing frames replaced at Costco ($87) last month, they have a little handheld electronic device that measures that. The two measurements (optometrist vs. costco) were 1.5 (millimeters) different.
I consider ordering online an iterative process. Just order a pair and see how it goes and learn from that experience.
I did this for the first time when I was in the USA last month. I had to order 2 pair (Zinni) and also add quick shipping option, because my time there is limited. From the front they both look great. But one of the glasses had rims that come out on my sides almost comically (like for a fat person whose cheeks stick out or someone with a very round head), and this wasn't obvious on the web site. They are basically unusable but I at least can use them as an emergency backup.
But for the price, I am very happy, and the other pair (which was really my second choice), came out great. The online Zinni system where you can picture yourself with the glasses worked pretty well. The lense prescriptions seem fine (near sighted plus astigmatism).
Also, someone has suggested to keep a record of the other measurements (like the total width, depth, of the frames etc.) so you know what works. For instance, the pair I liked would ideally be maybe a quarter centimeter wider (so where frame touches has less pressure on sides of my head), and I have recorded this for next time.
I have worn the same expensive glasses for a few years. The quality of the Zinni frames are not as high, but good enough.