When I was using 400 watt MH, I kept the lights about ~18" above the plants with new bulbs. You should take a reading with some kind of photography meter or something that can give you a number to base from and then keep tabs on the light level as the bulb ages....and move the light accordingly. As a general "old school" rule of thumb....If the light is casting a very sharp shadow under the plants, then the light is probably adequate. I'll probably catch Hell for advising such an inaccurate method...but, what can I say? This is what I was taught!
Also, I did not have any kind of lens or glass between the bulb and the plants, so the plants were getting the FULL spectrum of the MH, as well as full heat. I have been told that this is a very unsafe method to use due to hot bulbs getting a drop of cold water on them and BANG!!!....but...that's....what....I did back then!
You will likely have to play it by ear as far as adjusting the height of the light, because there is no universal formula that all plants will respond to the same. In general, when the plants are young, start the light farther away (~24") and then watch the plant response as they get older and larger and stronger and able to take more intense light. This will also apply to switching to a flowering spectrum.