When should you dip the headlights if you meet a truck or a bus on a hilltop?

If you meet a truck, bus or other type of high vehicle on a hilltop you must dip the headlights as soon as you see the identification lights on the vehicle's roof. The driver of such a vehicle is seated very high and would be dazzled if you were to wait to dip the headlights until you saw the vehicle's regular headlights.