There are many factors that need to be considered when it comes to the correct amount of photons (light) that is required for optimal growth. Usually people just look at the size of area that needs to be covered. More importantly what is the size of the canopy you are covering? Other factors that determine the appropriate amount light are: What strain or strains are you growing? What source of light are you using? (In your case 600 watt HPS. Are these single or double-ended?) Are you able to maintain proper VPD's? Are you supplementing CO2? In general cannabis is a light hungry plant. The most precise way to determine your light levels you will need to use a light meter that reads PAR. Flowering cannabis can require anywhere from 500 micromoles to 1000+.