Horizontal separation between characters or symbols should be between 10 and 65 percent of character or symbol height.
Additional Information:
Separation should not be less than 25 percent of character height when any of the following degraded conditions exists: (1) when character width is less than 85 percent of height; (2) when character luminance in less than 12 ft-L; (3) when luminance contrast is less than 88 percent; (4) when display is more than 35 degrees left or right of the straight-ahead line of sight; and (5) when the visual angle subtended by the character or symbol height is less than 15 minutes of arc.