How do you experienced drivers do this. A big truck passes you and you flash your lights to let him know he can come back over. Do you flash as soon as he clears or do you wait until you think he has enough safe distance in front of you. This would be subjective and the other driver may come over before you are ready for him too, with him thinking you aren't going to flash him. This happen to me many times.
Now what I do is flash as soon as the trailer clears plus about 10 feet. When I am flashed I flash thank you back and come over when I clear him by a distance I would be comfortable with and flash thanks again.
I guess what I am saying is the flash is to tell the passing driver that in an emergency he can come over.