Place yourself directly across from the person you are measuring. Sit at the same height so you are looking eye to eye on the same level. You can work across a small table or desk or place two chairs facing each other. Have the person put on the frames they are using for their new bifocals. Make sure the frame is sitting in a place that is comfortable and is adjusted on the bridge of the nose so the glasses will stay on properly.
Start measuring the right bifocal height first. All optical measurements are done with the right eye first, then the left. Have the person look directly into your left eye. Draw a horizontal line on the lens at the highest point of the person’s lower eyelid, sharpie recommended. This is placement of the top of the bifocal, now place a strip of scotch tape along that delineation to simulate the beginning of the bifocal, have subject look around, up and down, adjust tape accordingly for subjects comfort. At this point you will take your provided “PD ruler” and measure from the top of the scotch tape to the absolute bottom of the frame, with that number, Viola, you have your segment height.
Similarly, you’ll follow the same steps for a PAL, the reference marking will be from the center of the pupil versus just below.