Marking Bifocal or Gauge Reader Heights
With the mask fitted with the strap, mark the pupil as accurately as possible. Remove the mask and then recheck the markings. Many masks fit with the pupil at 5mm or more up from datum point.
As the diver about the position they would be in if underwater.
An allowance needs to be made for a more horizontal position.
Mark the segment position. As a guide, a segment height at 10-12mm below the pupil suits most divers. Keen photographers will tend to have the segment set a little higher from 6-8mm below the pupil.
Double check the segline position. Ask the patient to test the dive mask work in the normal position. Dive gauges or camera viewfinders need to be considered when testing the mask and adjustments accordingly made.