TT #197 · Parshas Tazria · Aliyah 5 · Passuk 31
English: If the priest examines the netek and its appearance is not deeper than the skin, and there is no black hair in it, he shall isolate the netek for seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:31
Torah Temimah cites Sifra (Toras Kohanim) for the rule governing what counts as spread into a netek.
Sifra — Vayikra 13, Midrash Sefer
English: The Sifra reasons: just as the “netek lesion” mentioned below is one into which the required spread does not extend, so too the “lesion” here is one into which that spread does not extend.
Textual derivation
The Sifra distinguishes the netek itself from the surrounding area in which spread is assessed. The bare area of the netek is not treated as a new spread into that same area.