Was wondering how many of you make use of the rule? I was doing a critique and mention the rule, got a question back on what was it. Probably will make for an interesting discussion.
For a sunny day (no pollution) at full strength with a front lit subject, the exposure will be f/16 and shutter speed will be the closest number to the ISO. It works. From there you can interpolate as needed for any combination needed.
Some of the cameras I used did not have a meter, Leica M2,3 &4 so it was the best way for getting the exposure. After a while you start figuring what to do for slightly cloudy, heavy cloudy etc. One thing to note is a variation with latitudes extremes, will make slight difference.
With the sophistication in meters today, don't think many will even consider but I feel its good to know. Does help to understand exposure.
al