The Yanks tested that with their C17's and found it was about 1000m away to hit the perfect spot:
According to the data generated by this month's flight and by earlier tests over Edwards, the ideal position for a drafting airplane is 3000 feet to the port side of the leading airplane. If there were a second drafting airplane, its ideal placement would be on the starboard side, about 6000 feet from the lead. This is pretty close to the tactical formations that large Air Force planes fly already, Erbschloe says, but this research will let the Air Force fine-tune the position of its planes to make sure they're riding the updrafts, not the downdrafts.
Source article
And another article from Aviation Week about it