TT #188 · Parshas Tazria · Aliyah 5 · Passuk 29
English: If a man or woman has an affliction on the head or on the beard,
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:29
Torah Temimah cites Sifra (Toras Kohanim) for applying the same separation to both measurement and spread.
Sifra — Vayikra 13, Midrash Sefer
English: Just as the two areas do not combine, they do not spread from one to the other.
Textual derivation
The separation applies both to measurement and to spreading.