Lights can be at any height as far as i am aware, the higher the better for me as i often clatter them with my cue.
Very slightly off subject, over here they have banned the selling of normal bulbs in favour of the more expensive so called energy efficient ones that take 10 mins to warm up, cost five times as much, are half the power and last half as long. Typically a lights canopy on a 12 footer has 3 100watt normal bulbs Gawd help us when they blow, theres no 100 watt "energy" bulbs being made.