I have a seperate pair of glasses for computer and sewing. I have the eye Dr. set the top area at the distance I am usually from the computer screen. Mine is approx. 26" from eye to screen. I have no idea what he does, but the glasses work great, for sewing I measure my comfort position and have hubby get the measurement to the needle. For me its approx 18". So the top of the glasses are set to 26" and bifocal area to 18". I just know it works, have no idea how its done.