I use 400's and I have mine about 10-12" above the tops of the plants. A good rule to check your plants is to place a hand over the tops. If it burns your hands it'll burn the plants, so lower the light until your hand can feel the burn, then heighten them again till you can't feel it. Usually with 400w Hps, this will be about 10-12".
At the moment your plants are having to stretch towards the light, don't worry I made the same mistake on my grow. My plants stretched so much they fell over. Luckily this site was on hand to save my crop. As soon as I positioned the lights correctly the plants stood right up again. Hardy little sods.