After my long stay in hospital I finally managed to shim the screen correctly. I tested it with my 2.8/60G Micro Nikkor in three ways (focus is on the 7 mark):
1. AF from tripod
2. live view precision with zoom
3. Manual focus with my F6J micro prism
Result: I needed both metal shims that came with the camera and original screen plus one of the two very fine plasic shims that came with the F6J screen. I sandwidched the plastic shim between the metals which seemed the most stable solution. The whole operation takes five minutes and with my D850. It features the new holding spring design, so I did need no tools except for my fingers plus a lot of patience.
Now my D850 ist a next to perfect tool for all of my photographic needs and wants including the use of manual focus with shallow DOF
