TT #186 · 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 the rule that head and beard do not combine for measurement.
Sifra — Vayikra 13, Midrash Sefer
English: Head and beard do not combine with one another.
Textual derivation
The affected regions are treated as separate areas.