Aquila
The best position for me is on the left side of the image, with the top of the monitor at eye level.
I’ve read that the natural resting gaze is slightly downward - in a relaxed state, the eyes don’t look straight ahead but about 10–15° down. Since this position is natural, it requires less effort from the eye muscles to maintain, as they tend to default to it.
The person on the right side of the image will have to lift their chin anyway to see the entire screen, since maintaining an upward gaze with just the eyes is less comfortable and can be more fatiguing over time (for people like me with BVD, whose eye muscles are already under constant strain, this can add additional stress). Also, when you look upward, your eyes open slightly wider, which can increase tear film evaporation. This may be important for people with dry eye syndrome.
Sunspark is right - a raised chin increases the load on the cervical spine. This can easily lead to reduced blood flow, headaches, and greater eye sensitivity due to increased nerve compression (if present). The same happens if you look directly at the center of the monitor, except the angle of chin lift is smaller than if you were looking at the bottom of the screen.
If the user is sensitive to bright light (like me), having the top of the monitor at eye level puts less strain on the eyes, as the display’s backlight doesn’t shine into your eyes from all directions. This feels more comfortable.
Also, if you have an older TN panel like I do, you shouldn’t look directly at the very top of the screen but slightly below it, so the bottom of the screen doesn’t fade due to the glow effect (I barely notice it, but it’s there). For a 24-inch monitor this isn’t critical, but for a 27-inch one it can be. This only applies if the display has a pronounced glow effect.
