TT #110 · Parshas Tazria · Aliyah 2 · Passuk 10
English: The kohen shall look [at it]. And, behold! there is a white se'eith on the skin, and either it has turned the hair white, or there is healthy, live flesh in the se'eith.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:10
Torah Temimah cites Sifra (Toras Kohanim) for the relationship between the whole source area and the whole affected area.
Sifra — Vayikra 13, Midrash Sefer
English: The entire relevant lesion must change the entire affected area; it is not enough for the whole of one area to change only part of the other.
Textual derivation
This unit requires a whole-to-whole change; a partial change on either side does not satisfy the stated condition.