At close range (50 - 100 meters) shoot 0.1 seconds ahead of him.
At further range (100 - 200), try shooting 0.1 - 0.2 seconds ahead
At very long range (200+), shoot 0.5 meters - 1 meter above him (quarter to half his height) and 0.2 - 0.3 seconds ahead of him.
Can't get much more specific than that. This assumes you have a ping of 100 milliseconds btw, adjust accordingly.
It may be hard to judge distance, but a general rule of thumb is that at 300 meters, you can barely see a few moving pixels on the edge of the view distance.