The easiest and most convenient method is the phone method. There are various phone-based solutions available that use the device's camera to measure your PD.
You can then follow the on-screen instructions, and allow the app to capture your PD measurement. It is important to follow the app's instructions closely and ensure that you are holding your device at the correct distance from your face for accurate results.
Driving the user out of your website and forcing them to download an app on their phone before completing their order is bothersome.