I am currently using 2 150w hps and have them about 12-15 inches from my plant. I was going to lower them but I've noticed that the plants are growing quite well at this distance. through the next 2-3 weeks that distance will decrease drastically and I may even have to move it up a few inches. you may have to try a few hieghts but I'd say keep it further first then move closer to avoid damaging your plants.