Great article and illustration from Car and Driver: How To: Adjust Your Mirrors to Avoid Blind Spots - Feature - Car and Driver
Been adjusting mine like this for years. Car Talk had an article way back when they first came online. I tried it, took a week or so to adjust. Now it's The Way when ever I get in another car.
Another way to do this: If you lean towards the door when adjusting the DS mirror (on LH drives, of course) and adjust so you can just see the edge of your car, then lean to the middle of the car and do the same for the pax mirror, you will adjust them almost perfectly to this standard. When a car leaves your center rear view, it should then appear in the side mirror. Mine are adjusted so if I can see half the car in my rear view, I can see the other half in that side mirror. Works like a charm! Thanks for posting! Not enough people do this, and it shows...
This assumes the centre mirror is usable - so that rules out Mini convertibles with no rear wiper....
I just keep a small bottle of Windex and a rag in the car, and clean the back window every other day or so. Unless you're driving off-road and kicking up mud or dust, you can see well enough out the back window to know if a car is there or not. I might try rain-x on the rear window to see if it helps at all with the rainy days...
when i bought my first MINI, the SA advised my on this technique. even though i had been driving for over a quarter of a century at the time, i found it educational and functional.