If you are to go the online route, please ask your optometrist for your PD when you get your prescription. That way you won't be measuring it yourself in which more than likely you'll get the wrong PD measurement.
When you do get them just walk into an optical and see if they'll check if the RX and PD was edged into correctly.
Also to the other guy who couldn't find his Rx, just call your Doctors office for a copy of it. Or if you're getting it locally, have the optical call them for you as the Dr's Office will always fax it to an optical when requested.