I can only relate my experiences and they are that lesser miles are needed if no stopovers are taken. Last year I flew bwi-dub-lhr-lun-jnb-lhr-muc-mad-opo-mad-mia-ord-buf-ord-dca all under 20K air miles(for the award). If you add those up point to point, that's a lot more than 20K air miles. Two weeks from today I fly dca-bos-snn-lhr-jnb-cpt using a oneworld award valid for 4k-9k air miles. A stopover (>24 hours) in london would have put my award in the next category. As far as I know, the reservation agent does not do the calculation of the air miles being flown, the 'computer' determines this. When are you headed off to S. Africa?