Besides height above the water, it's going to depend on how deep your tank is too. I have mine about 9" above the water (standard RMS mounts) over a tank that is 18" deep, and run them at about 45%. There are a lot of variables so you need to just dial it in for your particular situation. Best thing to do is either get a meter and aim for a certain intensity at a certain point in the tank, but even with no meter just slowly ramp up the intensity and keep an eye on whether things continue to look better or start to look worse.