So after many attempts, I guess I've found a "stable" way to have a good printing start allmost every time.
Here is my procedure if it can help, I guess it depends on the room temperature :
- Launch a print
- when bed reach it's temperature, around 70°C for PLA in my case, cancel it.
- wait it cool down for 5 minutes
- restart the print, bed temperature should be around 45°C, (If it's over 50° the print get stuck at auto leveling pass, just turn the printer off and then start again)
With that procedure I get the perfect Z Offset without tuning it each time, just one time
I think that with a thermal sensor gun device I should see exactly when the bed reach 45°.