If you have a cool tube then you can have it as close as 4" from the plant if not closer. If not, then I would say 12" away. Put your hand between the light and plant and feel how hot it is. Does it burn your hand or feel too hot? If so then its probably too hot for your plants too.
I run a 400 with just the standard reflector too. I did a decent amount of training to get an even canopy and i have it sitting at about 7-8 inches with a fan blowing just under the bulb and over the tops. My temps stay at 74-76. I would recommend getting a thermometer you can place near the tops, then just slightly raise/lower your light till you hit the sweet spot with your temps.