OK. I didn't write that with much precision. What I mean is assuming you are sitting comfortably upright in a neutral position and your gaze is level, horizontal and perpendicular to the floor, tilting your head back so that your line of sight is at a 5-30 degree upward angle is a recipe for eye and neck strain because your eyes have to work harder to relax and refresh. It is much more comfortable and natural for your line of sight to be slightly below level, several degrees or more angled downward.
So what this means for most people is: don't prop your monitor up on a stack of books or an additional stand, simply place the monitor on your desk and adjust the attached stand so that the top edge of the monitor is level with your natural, relaxed line of sight and then tilt the bottom of the monitor up very slightly as described by someone else above.
So what this means for most people is: don't prop your monitor up on a stack of books or an additional stand, simply place the monitor on your desk and adjust the attached stand so that the top edge of the monitor is level with your natural, relaxed line of sight and then tilt the bottom of the monitor up very slightly as described by someone else above.
Apple has a good resource on ergonomics: https://support.apple.com/en-gb/HT205655