Atlanta is 6 hours behind Johannesburg
Best overlap: 2:00 PM to 6:00 PM in Johannesburg is 8:00 AM to 12:00 PM in Atlanta.
If you set up a recurring sync, 2:00 PM to 6:00 PM in Johannesburg (8:00 AM to 12:00 PM in Atlanta) is the natural home for it. Atlanta is 6 hours behind Johannesburg, which gives you a solid 4-hour window. Book earlier in it and neither side is rushing. These times hold for most of the year, but Atlanta's daylight saving (which Johannesburg doesn't use) shifts them by an hour twice annually.