Where did you get "wear rate" from? The question was the amount of wear of the pads, not the rate of wear.
I don't know what type of 420 you have, but FWIW, I have a new set of rear and front pads for my 1992 400e right here in front of me. The amount of brake material for the rear is 3/8 of an inch. The front is 7/16 of an inch. The metal backing on both are 3/16 of an inch.
The mechanics probably did a visual "estimation". I don't have access to my CD to find out what it the recommended minimum thickness, but the wear sensor should let you know that.
Hope this helps