on the basis that you can buy the miles from BA at 1.7p each, I always try to do better than that.

i.e a round trip to the US in discounted J with BA is typically around £2k, or 100,000 points, so worth 2p a mile => use the points

If the fare is on for £1300 in a sale or going to Florida, you only get 1.3p a mile => spend the cash.

Of course the best use of miles is to actually spend them when & where you want. If you want to go to Geneva then by all means use miles - you might get better value redeeming them to S. Africa, but it's a jolly bad deal if you've no plans to go there in the first place!