Blackstone's Commentaries on Wife Beating...

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Read 'f' as 's'.    For the Latin, you are on your own.
Nothing about "rule of thumb" in any way, shape, or form.
"Chastisement" allowed in the same manner as over one's servants or children since "he is to answer for her misbehavior" but "the husband was prohibited to use any violence to his wife."

The RIGHTS of PERSONS.
Book I.
Ch. 15.
IN the civil law the hufband and wife are confidered as two diftinct perfons ; and may have feparate eftates, contracts, debts, and injuries ^c : and therefore, in our ecclefiaftical courts, a woman may fue and be fued without her hufband ^d.
BUT, though our law in general confiders man and wife as one perfon, yet there are fone inftances in which the is feparately confidered ; as inferior to him, and acting by his compulfion. And therefore all deeds executed, and acts done, by her, during her converture, are void, or at leaft voidable ; except it be a fine, or the like matter of record, in which cafe fhe muft be folely and fecretly examined, to learn if her act be voluntary ^c. She cannot by will devife lands to her hufband, unlefs under fpecial circumftances ; for at the time of making it fhe is fuppofed to be under his coercion ^f. And in fome felonies, and other inferior crimes, committed by her, through conftraint of her hufband, the law excufes her ^g : but this extends not to treafon or murder.
THE hufband alfo (by the old law) might give his wife moderate correction ^h. For, as he is to anfwer for her mifbehaviour, the law thought it reafonable to intruft him with this power of reftraining her, by domeftic chaftifement, in the fame moderation that a man is allowed to correct his fervants or children ; for whom the mafter or parent is alfo liable in fome cafes to anfwer. But this power of correction was confined within reafonable boundsⁱ ; and the hufband was prohibited to ufe any violence to his wife, aliter quam ad virum, ex caufa regiminis et caftigationis uxoris fuae, licite et rationabiliter pertinet ^k. The civil law gave the hufband the fame, or a larger, authority over his wife ; allowing him, for fome mifdemefnors, flagellis et fuftibus acriter vering him, for fome mifdemefnors, flagellis et fuftibus acriter verberare uxorem ; for others, only modicam caftigationem adbibere ^l.
{FS}

Christina Hoff Sommers is correct.