Johannesburg is 6 hours ahead of Washington, D.C.
Best overlap: 8:00 AM to 12:00 PM in Washington, D.C. is 2:00 PM to 6:00 PM in Johannesburg.
8:00 AM to 12:00 PM in Washington, D.C. is when Johannesburg is also working, at 2:00 PM to 6:00 PM their time. You've got a decent 4-hour window. Set a recurring time near 8:00 AM in Washington, D.C. (2:00 PM in Johannesburg) and stick to it. Just remember Washington, D.C. changes its clocks for daylight saving and Johannesburg doesn't, so confirm the overlap whenever the seasons turn.