Avraham Sacks asks:
How does R' Yehudah learn from Mitzcho (instead of Metzach) that if the tzitz is broken Lo Meratzeh?
Avraham Sacks, Ramat Beit Shemesh
The Kollel replies:
Shalom R' Sacks!
I see that the Torah Temimah (Shemos 28:38) concludes that the basis of the Drashah is this: If the word "Metzach" alone had been used, then it would not preclude the usage of a broken Titz. The reason being because there may be a small forehead that the broken Titz -- despite its reduced size -- would indeed fit upon; for example, that of a child. But once the Torah say "Mitzcho", that means it specifically has to fit the forehead of the Kohen Gadol, which it can do in full measure when it is complete.
I hgope this helps!
Best wishes,
Yishai