The smallest bubbles rule was just a rule of thumb. The idea was that each time you exhaled you just never wanted to be going faster than the tiny ones escaping your exhaust tee. As bubbles get bigger they'd expand and be going faster than what was considered safe. The concept was no way an accurate measurement of one's rate of ascent.
As far as accurate, a new diver using a watch and depth gauge, or a computer with slow-right-too fast is not going to be all that accurate either. Because one uses a more accurate tool does not mean one is more accurate.
The reason the rule of thumb does not work is that we no longer use 60'/min, so it is no longer taught. Laws of nature determine how fast bubbles rise in water, once you find the right size bubble to follow it can be as accurate as any other method. When there were other things to attend to on the ascent, it was nice to know that you could follow the bubbles at the same speed as following your depth gauge and watch.
I believe in the last couple of years there was a discussion of what the actual size of the bubble was, and a bunch of math, but I haven't found it yet. Probably in vintage, where a lot of "lost to history" techniques and gear are discussed.
The technique is to follow a small bubble up and as it increases in size refocus to another bubble the same size as the original. I'm thinking a couple of mm, but it's been a while. After a bit you get a feel for it, same as following gauges or computers.
Bob
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"No matter what happens, somebody will find a way to take it too seriously" -Dave Barry