The best way to see it is to manuver for a 90deg crossing shot or diving down higher from the front. If you pull the trigger when the pipper and cross match you miss. If you lead the cross with your pipper so when you pull the trigger early, the cross passes through your pipper 1-2 sec later, the con flys into a stream of rounds.
So after some amount of time working with the lead comp sight green crosses in offline mode, I think the green cross is an indicator of the point in front of the con a stream of rounds need to pass through so the con will fly into them. Dead 6 shots inside of the rear 30deg cone work with the pipper dead on the green cross for the most part.