Please help - Lighting Question - 300w Mars Hydro NL Auto Grow in Jardin Tent

Please can someone help me. I am unsure how high to have my leds above my small seedlings - I want maximum growth.. at the moment it is day 6-7 and they are sprouting 2nd true leaves set but I want to know how high people have their LEDs above their plants? Any suggestions, of course experienced...