Good time in California is calculated and applied at the start of the sentence, already baked into the first projected release date the inmate sees. It is not something that accumulates over time through behavior. The system grants a set amount of good time credit upfront, and the inmate can only lose it by picking up disciplinary infractions. They cannot earn more than what was granted at the beginning.
For most serious and violent offenses in California, inmates are required to serve 85 percent of their sentence. Good time does not reduce that floor beyond what was already calculated.
What education does is something more valuable than shaving days off a sentence. Earning a high school diploma or pursuing college coursework while incarcerated is one of the strongest indicators of successful reentry. It changes what someone is capable of when they walk out, gives them credentials that translate to employment, and signals to parole boards and classification officers that the person is investing in their future. Across the board, inmates who pursue education are significantly less likely to reoffend.
So while a GED or college degree may not directly reduce the calendar, the return on that investment after release is real and documented. Encourage your person to pursue every educational opportunity available to them. It is the best use of the time they have.