Sunny 16 used to work fine for me here in Devon, as long as there weren't many clouds. If it was Sunny with clouds, I went for Sunny 11, normally relying on exposure latitude. I don't need to do that anymore now though, seeing as I got a Sekonic L-8 meter. It's extremely reliable, and dead accurate. Highly recommeded for fast, easy, and accurate metering. There's also a free PDF Manual knocking about in the depths of the internet.
I grew up on the Sunny 16 system and still at 63 use it before turning on the meter if I have it with me. With practice, you eye and mind can follow light and quickly judge the adjustments starting with the rule but I seriously doubt many have made a project or set a goal of learning the system. I used to teach it to my 1st year photogrpahy students who generally as a group quickly grasped the concepts as thye had not be trained to rely on a meter as of yet.
If you want a good, actually excellent, system chart go find an older Rolleiflex that has the latitude and time adjustments on the chart for the system. In testing it I found it was a good refinement of the general rule but possibly because of changing atmospheric conditions and the ozone layer just provided a better jumoing off place to start.
I think the largest issue with the system is one similar to using a meter which is what is the true speed of the film. It seems to me at least that with older film the listed speed seemed closer to its ideal; maybe a reult of more subjective testing back them. If the listed film speed is a stop or more off from the ideal you have to adjust accordingly and why you may have some error introduced with the system. An iso 100 film better rated at iso 50 will throw off the system by an entire stop and it is not unusual for todays' films to be that far off. Maybe the makers are simlply relying on the film latitude to make up the difference and they "need" a certain listed film speed for marketing reasons.
I seem to remember Fuji (I remember it being Fuji) at 1 time published a modified Sunny 16 guide in one of its color film cartons that was a Sunny 11 syetem. A fellow photogrpaher who knew film better than I told me it was because the listed speed was not the same as the optimal speed and hence the maker modded the system on the box to advise buyers to set their metering to the preferred optimal speed. I guess a way to get a specific film speed to market for the marketing guys while telling those smart enough what the photographers what it really should be shot at.
I remember when Verichrome Pan started showing up marked asa64,over time it magically became asa125. But the film still worked very well at asa64. This film was coated with three emulsions, like Tri-X, and very flexible.
You put it into a start position then rotate it depending on time, time of year, film speed, light and scene.
To try out the method without having a disc, I have converted it to a series of adding and subtracting sequences as shown below:
Start with a number depending on ISO used:
ISO 25 +8
ISO 50 +9
ISO 100 +10
ISO 200 +11
ISO 400 +12
ISO 800 +13
ISO 1600 +14
Add a number depending on the light conditions:
Strong sun with white clouds +4
Strong Sun +3
Weak, hazy sun +2
Very dull 0
Add a number depending on the time and date:
May to August, 10 am to 3 pm +4
May to August, 8-10 am, 3-6 pm +3
May to August, 7-8 am, 6-7 pm +2
September, October, March & April, 10 am to 3 pm +3
September, October, March & April, 8-10 am, 3-6 pm +2
September, October, March & April, 7-8 am, 6-7 pm +2
November to February, 10 am to 3 pm +2
November to February, 9 to 10 am +1
And finally, subtract a number depending on the scene:
Open sea and sky and scenes from the air 0
Distant landscapes and beach and snow scenes -1
Open landscapes and scenes with light foreground -2
Groups in the open and near views of houses and trees -3
Distant buildings and wide streets -4
Scenes with heavy foreground and near landscapes -5
Close up portraits in the shade and scenes in heavy shade -6
Bright interiors -7
Dull interiors -14 (disc says rotate twice)
As a test, put in some standard sunny 16 settings:
ISO 100, start with 10
Bright sunny day, use strong sun setting adding 3 = 13
For mid day in June, add 4 = 17
For an open landscape, subtract 2 = 15
The numbers are EV or Exposure value numbers. EV 15 is 1/125 at f16 which is what sunny 16 recommends for these conditions.
I think the disc will be out by one stop at longer shutter speeds as it uses the older sequence of speeds which has six stops between 1/2 and 1/250. i.e. 1/2, 1/5, 1/10, 1/25, 1/50, 1/100, 1/250 whereas a modern sequence will have seven stops. i.e. 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250.
Or perhaps this extra stop compensates for the one stop change film manufacturers introduced, probably after the disc was made. either way, I know the disc works as I have used it instead of a light meter many times.
Wow, that's a trip down memory lane, Unfortunately I'm old enough to have bought one of these new, it must be about forty years since Johnsons stopped making them .