When should you dip the headlights when overtaking?

When overtaking in the dark you should try to keep the full beam headlights on for as long as possible, and scan the road as far ahead of the car as possible, before dipping the headlights.

When you are close enough to the vehicle you are going to overtake that your full beam headlights hit it, it is time to dip your headlights. Switch back to full beam headlights again just before the moment of overtaking, approximately when the front to your car is about to pass the rear of the vehicle you are overtaking and you are no longer at risk of dazzling the driver.