Charging vs Blocking

[Splute]

What is the "rule of thumb" that seasoned officials use when judging to call "charging" versus "blocking"? In the games that I have watched it seems to be split about 50/50 going each way on the calls that are really judgement calls; meaning not obvious (in my mind) one or the other. It seems that if A1 (offense) leads with body or shoulder, it is most often called "charging". I would agree most of the time if defense has LGP. If A1 is airborne, it is usually called "blocking" regardless if B1 has LGP. Could someone elaborate on what you use to go by?