TT #196 · 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 minimum gris measure for a netek.
Sifra — Vayikra 13, Midrash Sefer
English: Just as an ordinary skin lesion must meet the minimum measure called a gris, a netek must meet that same minimum measure.
Textual derivation
The Sifra applies the standard minimum halakhic measure—a gris—to a netek as well as to an ordinary skin lesion.