Johannesburg is 6 hours behind Shenzhen
Best overlap: 2:00 PM to 6:00 PM in Shenzhen is 8:00 AM to 12:00 PM in Johannesburg.
If you set up a recurring sync, 2:00 PM to 6:00 PM in Shenzhen (8:00 AM to 12:00 PM in Johannesburg) is the natural home for it. Around 4 hours of overlap is enough to work comfortably, as long as meetings don't drift to the end of one team's day.