I think this is the setting you need to change: DAMPER_GAIN=1.0 ; Set to 0 for Logitech g920
In the file assetto_corsa.ini C:\Program Files (x86)\Steam\steamapps\common\assettocorsa\system\cfg
The rest of the settings is simple. You can turn off the ffb effects or keep them on if they don't bother you. And keep gain% at 100, min.force try with 0 and then try with 10%.
Then when you're on the track you might need to fine tune the force% per car, if is too strong. For that use + - buttons from numpad to adjust ffb force per car if you need.