Well for one, low mileage awards are very hard to come by on this route. Not sure when OP is looking to leave, but a quick check of the Award Calendar (I know, not the most reliable tool in the world) found only a handful of low mileage dates searching for one ticket:

ATL-JNB: couple of random dates in March
JNB-ATL: a few dates at the end of December, a couple more at the end of January

I guess it depends on OP's stash of miles...