A rule of thumb is not intended to be strictly accurate or reliable for every situation. It is an easily learned and applied way of approximately calculating some value.
The exact origin of the phrase is uncertain. It likely refers to one of the numerous ways that thumbs have been used to estimate things – judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brewing beer, measurement of an inch from the joint to the nail to the tip, or across the thumb. The phrase joins ‘the whole nine yards’ as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down. The earliest citation comes from J. Durham’s Heaven upon Earth, 1685: "Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb."
The term could have originated, for example, with carpenters who used the width of their thumbs rather than rulers for measuring things, cementing its modern use as an imprecise yet reliable and convenient standard. Another possible origin comes from farming fields. Plants need a fairly precise depth to seed properly, whether planted from seed or being replanted, but the depth can be estimated using the thumb.
'Rule of thumb' has been said to come from the belief that English law allowed a man to beat his wife with a stick as long as it is was no thicker than his thumb. Judge Sir Francis Buller, who is reported to have made this ruling in 1792 was notoriously harsh in his punishments and had a reputation for arrogance, but there's no evidence that he ever made the ruling that he is infamous for.
Despite the phrase being in common use since the 17th century and appearing many thousands of times in print, there are no printed records that associate it with domestic violence until the 1970s, when the notion was castigated by feminists.
I like ‘rule of thumb.’ It’s how I do most of my cooking. Pour a little salt in the palm of my hand, eye it, and decide, yes, that’s a teaspoonful! Much simpler than actually measuring.