There are 2 schools of thought.
One is put your chin at the top of the catcher’s head and maintain the same head height on every pitch.
The other is put your eyes at the top of the zone and adjust to the height of each batter - get your eyes under the batter’s elbow. This is what I do. If my eyes move up, the pitch is a ball. My eyes do not strain on a low pitch until about 2 balls below the knee. My Trackman reports reflect that I’m getting those pitches correct.