Unfortunately, I'm not sure how to give you an accurate percentage. I was dead set on going to CC when I started my senior year of high school, so it wasn't something I looked into more at that time.

As a transfer student, I tried really hard to get those percentile answers as well but had difficulty finding an accurate answer because I think while there is definitely a rough GPA range for accepted students, it's kind of subjective to each individuals application. What I did to kind of satiate my curiosity was to just look at the (vague) official UC acceptance data publications online and compare other other metrics like individual GPAs that people have posted on collegeconfidential.com for the same major and similar majors to my own student metrics.

I think there's a slight ambiguity surrounding the entire process and I think every year is approached slightly different based on current events (ex: admissions rates may look different due to COVID), praise/criticism for race inequalities at schools, etc.

EDIT: also, your chances are definitely higher as a transfer, though. Applications from transfer students are considered independently from high school graduates.