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