Adjust your height so that the center of gravity of the head (at eye level) matches the sturdy part of the headrest.
Do not place it lower, it can aggravate the injuries. Do not lean on it, it is an element of security, not comfort. Make sure it is properly locked.
This piece is published by A Class Driving School in Bristol which provides qualified and DVSA approved local driving instructors. It also provides a range of regular classes and intensive driving courses in both manual and automatic vehicles.