The sunny 16 rule does work, It is not the zone system. But if you are in the field and your meter or battery dies it will get you a workable negative as opposed to coming home empty handed
I spent the first week in Navy photo school in the 50í memorizing the sunny 16 rule because as a general rule we didnít have meters available all the time.
Based on sea level it breaks down as follows using the film speed as the shutter speed Bright sun ( you can not look at it with your naked eye)harsh shadows f/16
Haze sun you can look at it soft shadows open up one stop (f/11)
Cloudy Bright you know it is there somewhere but no shadows ,open up 2 stops f/8
Cloudy dull getting really dark open up three stops f/5.6
Open shade open up one stop from normal f/11
Closed shade open up two stops from normal f/8
Sidelight open up one stop f/11
Backlight open up two stops f/8
Kodak use to publish the Master Photoguide with great exposure dials for just about any condition, I still have mine and once in a while check my expensive and sophisticated meters against it when in doubt.
The Sunny 16 rule doesn't work in the UK or I suspect most of Northern Europe