the best 'rule of thumb' for light height is to posistion the lights as close as u can without frying your girls. u also have to be careful of whats called 'light bleaching', thats when the lights overwhelm your plants. i use the following system:
1. place your hand palm down over the top of your plant(s), lower the lights until it is uncomfortable to the point that u cant leave ur hand there for at least 30 seconds.then raise the light 2 or 3 inches and that should work. DO NOT let a HPS or MH bulb touch your skin when they are powered on!!! u WILL get burnt and it may even cause the bulb to explode. hope this helps...