Hold the back of your hand under the lamp to feel the heat, wherever your hand is comfortable and not burning is about the height you want your lamp above the plants. Because your plants are only young, add about another 6 inhces or so to be safe. I'd say keep the lamp at least 18 inches - 2 feet away. Is your lamp HPS (high pressure sodium), or MH (metal halide)? How come you only have 2 plants growing under 400w? That's a lot of electricity to be using for only 2 plants. What is the temperature inside your grow space?