using the AVL 21 +-1.5 deg BTDC @1000 rpm is spec. (with clamp near injector and lamp onto balancer). (for SUN and some others it is 18 deg)
For my AVL I also got a cable connection to the diagnostic socket instead of the lamp. Using this I can see the approx. 25 deg BTDC on the display of the AVL 875.
With the lamp I saw the 21 deg.

There are no specs for the advancing device.
If there is approx. 10 deg advancing @2000 rpm then the device is o.k.
The Bosch guy said it is only black or white, nothing in between.
No advancing means broken (noisy) or blocked movement by a wrong screw, but no wear or weak springs.

Tom