Light distance from seed starting tray?

I gotta 400w hps in a grow tent with no cooloing tube or anything except some ventilation. How far away should my light be from my seed starting tray and when should I move it to about 16-18in away? Please help!

i have lowrider and masterlow if that matters

well it depends on how old the seedlings are , if they have just broken the soil i would keep my 400 about a metre away and move it in closer each week. move the light 16-18 inches away when the plants are around the 2-4 week mark