Just happened across the Rules of Thumb website, a database/catalog of rules of thumb.
It's an interesting site, and an interesting idea, and it grew out of thirty years' worth of collecting rules of thumb.
But the site seems to have a somewhat different idea than I do of what constitutes a rule of thumb.
For example, although their main collection includes many items that fit my idea of a rule of thumb, it also includes items like this one:
Never tell a child how a used-car dealer prices automobiles, how a butcher makes sausage, or how a Texas politician makes money.
To me, that sounds more like a proverb or a joke (and, really, a variant of the old line attributed to Otto von Bismarck about laws and sausages) than a rule of thumb as such. I'm dubious about whether advice counts as a rule of thumb in general (see below), but even if you allow advice, this one isn't even really advice--presumably the person who submitted it wasn't really saying that you should never tell a child those things.
To me, a rule of thumb is a way to easily guess or approximate or estimate or predict something, usually something to do with numbers or values.
For example, it can be a way to approximately measure something: their About page says that the number of times a cricket chirps in 15 seconds, plus 37, is roughly equal to temperature in degrees Fahrenheit. Or a way to approximate how much of something you need: another one from the "children" section of their main collection says that, when planning a party for kids, you should have "One adult for every five children under age five. One adult for every eight older children."
My dictionary suggests that a rule of thumb can also be "a general principle regarded as roughly correct" or "a method or procedure" based on common sense. I think I'll buy the "general principle" part, but I'm a little dubious about the "method or procedure" thing; for example, I don't know if I would consider this one from the site to be a rule of thumb per se: "If you don't want a cat to jump into your lap, don't make eye contact with it." I would call that advice rather than a rule of thumb. But apparently most people consider "measure twice, cut once" to be a rule of thumb (in fact, that's what led me to the site), and that's more advice or a procedure than it is a way to estimate a value, so maybe my definition is out of step.
Anyway. Regardless of my nitpicks about definitions, it looks like a neat site, and it does contain plenty of items that are indubitably rules of thumb even by my definition.
(P.S.: 45 minutes after posting this, I happened across an old entry of mine that refers to a piece of advice about a procedure as a "rule of thumb." So I guess I do use the phrase that way. But in that kind of context, I think I specifically use the phrase to mean "if you don't have any particular reason to do the process in some other particular way, then it's a fairly safe bet that this will be a good general approach to take" rather than (say) "here's the procedure you should always follow." But I may recant this, too, after further thought and more examples.)