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
OP is talking about at least 2 stops of error. While each meter might have differences in calibration and behavior, I doubt they should be THAT far off if calibration hasn't drifted. Besides, OP is talking about a meter he acquired for free and I assume he doesn't know the history of previous usage. Anything is possible here. Since we are talking about a measuring instrument, my first step will be to veify its reading with known good standard. (or at least another meter) Then, I would proceed with becoming familier with the unit.
By the way, I have more than two dozen watches. While none of them are accurate to seconds to each other, all of them are within few minutes to each other. To me, that's good enough to be usable for daily life.