Hot to use / finetune UBL (no replies)

Hi,

so I use these commands to level the bed using the BL touch probe:

G28 
G29 P1
Mesh invalidated. Probing mesh.
Probing mesh point 1/9.
Probing mesh point 2/9.
...
Probing mesh point 9/9.
G29 T
Bed Topography Report:
    (  5,220)      (200,220)
        0       1       2
 2 | -2.099  -0.667  -0.043
   |
 1 | -0.464  -0.105 [+0.365]
   |
 0 | +0.153  +0.377  +0.664
        0       1       2
    (  5, 15)      (200, 15)
G29 S0 -- save to slot 0
G29 A -- activate
M500 -- save everything

This works just fine. Repeatability can be tested using G29 K0 and it's below 0.01 mm if I do not touch anything between the probes.

But now comes the Z offset. I think I am doing something wrong here.
I use M290 babystepping to finetune the Z offset. But it somehow varies a lot.
I only get good results if I do a G29 P1, then immediately the Z offset and then printing.
If there is anything between printing and offset it just is off again.

G28 Z -- so it moves the head to the center of the bed
-- put paper under the head
G1 Z0 -- move head down to where printer thinks is zero. 
M851 -- check settings
  M851 X-45.00 Y-5.00 Z-0.8 ; (mm)
M290 Z-0.1 -- or similar until I just feel some resistance moving the paper

M500 -- save again

before every print I do homing and 3 point leveling to have the bed's tilt.

G28
G29 L0          ; Load UBL mesh values from slot 0.
G29 J           ; Probe 3 points and tilt the mesh to the plane.

Should I save after G29 J?

PS yes I tried leveling with heated bed as well.

Marlin
2.1.x Bugfix from current bugfix branch.

Hardware:
Creality Ender 3 Pro STM32 427 BLtouch
Creality CR10-S5 STM32 427 BLtouch


thank you very much