it seems to me that the "rule of thumb" was something already in common parlance, so the phrase doesn't originally refer to the switch as thick as a thumb myth. this section, below, seems most likely to present the most balanced view. as someone who leans toward foucauldian analysis, i suspect, too, that the references to the story as, possibly, a 'reverse formation' to justify increasing use of dv is likely. but, just a gut instinct...no firm commitment to it. At any rate, here's the section to which i was referring in my first response to you.
"What Blackstone says is that moderate violence used to be allowed, but not after the time of Charles II, though the lower rank of people still claimed it as a right; but the courts did still allow husbands (of all ranks) to restrain their wives in cases of gross misbehavior.
I document all this in an article, "Rule of Thumb and the Folklaw of the Husband's Stick," to appear in the September issue of the Journal of Legal Education. I unpack all of Blackstone's sources, including Roman civil law, and analyze the American cases in which thumb-measurements or other criteria for the husband's stick are mentioned, and I also deal with an English judge of the king's bench, Sir Francis Buller, who in 1782 was lampooned for his view that husbands could use a smaller-than-thumb stick.
Andy Kelly (aka Henry Ansgar Kelly, English Department, UCLA) http://englishwww.humnet.ucla.edu/faculty/kelly/