DO NOT MAKE THIS MISTAKE WITH YOUR PUPILS!!!!
Most driving instructors still use the outdated and frankly wrong method, of telling their learners to adjust the mirrors, so they see half sky/Half ground.
This is wrong and 90% of the time just will not work!!
Depending on where the car is parked and the angle of the road, it is wrong to try and line up the mirror using this wrong method.
As you can see in the picture on the left, the mirrors is set correctly, you can see for a good distance behind the car, what you cannot see is half the mirror set to sky.
You need to set it with something that works no matter where you are or what is behind you. - It is much better to adjust the mirror and find a focal or reference point on the car - in this case the front door handle in the bottom left corner of the mirror.
For the passenger door, it would be the front door handle in the bottom right corner of the mirror.