Cfls you can be pretty close. As Lee Harvey stated, that is they way I test any light system. Put your hand at canopy height, hold it there for awhile. It you feel a kinda stinging uncomfortable heat, you have it too close. This will work with any light systems. Also watch your plant if it begins to strech too much lower your lights. Most plants are grown best at aroung 75 to 80 degrees. Good Luck