I've been doing this since the 90's.
It was odd at first but it really makes complete sense.
The key is adjusting it on the highway as you let cars pass you. Watching them in the rear view mirror slowly disappear and then finally starting to appear in the side mirrors. The trick is to have just a little overlap between the rearview and sideview mirror.
I remember showing this to an ex-girlfriend after I had to adjust her sideview mirrors when she wanted me to drive her car on a trip. She was shocked and thought I had discovered Noah's Ark.
In high school driving school, they taught us wrong.