When lighting a sign with ground-mounted floodlights, there are three considerations:

Setback

When using ground-mounted floodlights to light a sign, the recommended setback is a distance equal to 3/4 the sign height.  For example, the setback distance for a 12 foot by 6 foot sign would be 4.5 feet.  Locating the floodlight closer will sacrifice uniformity while setting it farther back will result in a loss of efficiency.

Setback distance = 3/4 x sign height
3/4 ( 6 ft.) = 4.5 ft.

Spacing

The rule of thumb for spacing floodlights is not to exceed two times the setback distance.  If the setback is 4.5 feet, the floodlights should not be placed more than 9 feet apart. 

Spacing = 2 x setback distance
4.5 ft. x 2 = 9 ft. apart

Aiming

The floodlight should be aimed at least 2/3 up the sign.  For example, if a sign is six feet tall, then the floodlight will be aimed approximately four feet high.  After installation is complete, aiming can be adjusted to produce the best appearance.  Mounting a full or upper visor to the floodlight can reduce unwanted glare. 

Aiming point = 2/3 x sign height
2/3 (6 ft.) = 4 ft. high