Exactly wat ECU-MAN said

I did mine a week or so ago in a friends garage... it was night time, reversed out about 5 metres n turned the lights on, i dind't have anything to measure it against (no chalk :P), jus lifted them a lil higher so i can actually see in streets with minimal lighting at night..

But wat ECU-MAN said is the best way, follow his steps and you cant go wrong