With open bulb MH, less than 18" seems to cook them. I keep a graduated 24" stick handy for checking things. So, MH I keep around 20" and HPS around 18". My cool white T5s in my cloning cabinet can be within a couple inches...let them grow into the light and it too will bleach. Last grow I augmented my HPS with T5 blues and while I think I could have gone closer I kept it at the HPS height.
