Apple iMac (21.5-inch and 27-inch, Mid 2011) - Manual (Page 73)

73 Chapter 5 Last, but Not Least
Display
Arrange the display so that the top of the screen is slightly below eye level when you’re
sitting at the keyboard. The best distance from your eyes to the screen is up to you,
although most people seem to prefer 18 to 28 inches (45 to 70 cm).
Position the display to minimize glare and reflections on the screen from overhead
lights and nearby windows. The stand lets you set the display at the best angle for
viewing, helping to reduce or eliminate glare from lighting sources you can’t move.
Thighs tilted
slightly
Shoulders
relaxed
Screen positioned to
avoid reflected glare
Forearms and
hands in a
straight line
Forearms level
or tilted slightly
Lower back
supported
Feet flat on the floor
or on a footrest
Top of the screen at or
slightly below eye level
Clearance under
work surface
18–28 in. (45–70 cm)