The "Seg" height is the distance from the bottom of the lens to your pupil.
We use this to determine how to make progressive, bifocal, and trifocal lenses.
Put on any pair of clear eyeglasses.
Face the camera directly.
Snap a picture.
(It helps to be close up to the camera, and in a well-lit area.)
Email the photo to customer service with the frame size information.