In the valley of the blind, the one-eyed man is king.
This is the same chart that your doctor uses to determine your visual acuity. The normal height for the letter A is 88 mm, and the viewing distance is 6 meters (~20 feet).
To properly view the chart on your monitor:
For example, if the top letter "A" is 60 mm high, the following are the acuity distances to clearly see the lines below
WHEN YOU ARE NOT WEARING YOUR GLASSES:
If you wear glasses with the correct acuity prescription, while wearing your glasses you should clearly be able to see the small 20/20 letters at 20 feet (6 meters), but NOT clearly see the 20/15 letters unless you are far sighted. Likewise, you should be able to see the 20/15 letters at 15 feet (4 meters) and the 20/40 letters at 40 feet (12 meters). The distances in the right columns for viewing the letter images without your glasses (where you are able to barely discern the identity of the letters) indicates your actual refraction.
(Note that the display height of the images are a percentage of the top height letter "A" and need to be factored for your specific monitor's display. If the top letter "A" of your display is 50 mm high rather than the 60 mm in the table, then the viewing distances should be 8.3% shorter.)