No additional mirror is needed if the stock mirrors are properly adjusted. These additional mirrors can even cause harm:
1. They display a distorted image which makes it hard to gauge the distance of the other car.
2. You now have more mirrors to check which means more focus in unwanted directions.
3. Adhesive mirrors are placed on some of the glass of the original mirror, thereby creating new blind spots.
4. Mirrors mounted ontop the original mirrors can create a blind spot in front (in junctions and roundabouts) and increase aerodynamic drag.
Open up the driver's side mirror untill you just CANNOT see the edge of your car, and than one "click" (with the electronic adjustment) further out. Adjust the passenger's side mirror untill you just cannot see the edge of your car, and than three "clicks" further out. When you park/reverse, readjust the mirrors to fit.
By this adjustment, you have effectivelly removed your blind spots!