University of California Counselors described how this is done.
Basically, your UC-recalculated GPA is compared with the top 9% threshold UC-recalculated GPA from a recent previous class at your high school (high school submits top 15% academic records for UC recalculation of GPAs and setting of the top 9% threshold).
You may want to ask your counselor if they know what the threshold GPA is for your high school.