measuring brake pad thickness

[sb02x5]

I've Bentley x5 manual, and in it, the instruction for measuring the pad thickness uses a special tool 34 1 260.i don't have this tool but have a generic dial calipers.

looking at the oval opening on the rear left outer brake pad, there appears to be a ledge below the metal backing. is this the very top of the pad?
is the measurement taken from that ledge to the surface of the rotor?
The drawing in the manual shows the special tool setting on the outer metallic surface and the plunger at the rotor.
if this is the case, does anyone know how much the metallic backing plate thickness is?
thanks in advance

[X5 Meister]

Can you post the page from the Bentley to show this better. Here is the tool and I'm curious how it works.

[sb02x5]

I don't know what constitute copyright infringement so I will verbally state the diagram.
the pin area of the tool is resting on the oval rim at the outer metallic surface of the pad and the tip of the needle is hitting the rotor surface.
with this setup, you are to read the indicator on the tool.
if you push the rod into the tool flush with the surface of the pink sleeve, what is the reading?
or conversely, if you move the plunger such that the tool registers 3 mm, how much does the needle stick out
thanks