A good rule of thumb is to put the brace at 2/3rds the height of the fence. This will allow for 1/3 of the tension to be above the brace and 2/3rds below. The other consideration is the distance between brace posts, which should be at least two times the height of the top (i.e. if it's a 4 foot high fence, the minimum post spacing is 8 foot, although 10 feet would be ideal).