Ok first off, seedlings are not going to want a hps over them. You should use some 6500k cfls. You can get these at any walmart or lowes. Once your plants are a few weeks old, you can put them under the hps. My 150w hps recommends that it be placed 18-24 inches above the plants, but this all depends on how good your temperature control is in your room. Use your hand as a reference to measure the distance from the lamp. If it's warm to your hand, it's too warm for your plants.