Mutt,
If you look at your brake wear indicator pins you will see a flat metal angle bolted onto the brake unit with a metal placard( if its still there!!) on it.
The max brake wear is when you reach the end of the angle bracket.
Originally max wear was right down to the edge of the brake actuator piston housing.
The bolt on angle bit was the AD which effectively reduces the amount of wear you can have.
(!!! what angle bit ??????)