TT #624 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:02
And it shall be, if the wicked one is to be flogged, the judge shall cause him to lie down and be beaten in his presence, according to his guilt, by number.
Torah Temimah points to Makkot 23a.
TT lemma: כְּדֵי רִשְׁעָתוֹ בְּמִסְפָּר.
English: They flog him one-third in front and two-thirds from behind, for Scripture says ‘and he shall strike him before him according to his guilt by number’ – one measure of guilt in front, two measures of guilt from behind.
Derash on the combination of ‘before him’ and ‘by number’ (Bavli) — the dual language yields the 1:2 ratio.
Of the total number of lashes, one-third fall on the front of the body and two-thirds on the back (shoulders). The number itself must be divisible so that this proportion can be observed.