How close 600watt hps bulbs can be above plants without burning and yield

i have 9 plants under a 600watt bulb wondering how close i can get the light to the top of the plants without burning them and does closeness of light affect yeild

18'' should be a good distance

the yield will be good and the bud will be tight
here ya go:leaf

thx 4 da help im gona do anutha thread in dis category wanna luk it up n help me out thx