Eye Ergonomic tips
The top of the screen (not the top of the monitor itself) must be at or slightly below eye level
When looking at the middle of the screen, your eyes should look only slightly downward
Tilt back the monitor 10 to 20 degrees. If you don't carry around a protractor, that is ok. Have the bottom of the monitor just above the keyboard.
A good rule of thumb: Keep the monitor at an arms distance. You should barely be able to touch the monitor with an outstretched hand. But the larger the screen, the more distance you will want.
Why this rule of thumb works:
Try it for yourself. Try the finger to nose test. The closer your finger gets to your nose, the more eye strain you get. So it makes sense to keep the monitor farther away from your eyes.