I’ve been setting my z-offset manually using the paper test then tuning a test print. I usually have to crank the z-offset down an additional 0.05-0.1 mm to get acceptable results. The paper test seems very subjective as to the paper thickness and judging the correct amount of drag.
Yesterday I discovered by reading the online documentation (yes I know - RTFM) that the auto z-offset function uses a strain gauge to detect when the nozzle touches the bed. This should give me a repeatable, objective starting z-offset number. In fact, the number the automatic measurement gives me (-2.67mm in my case) agrees very well with the paper test if I accept a fair amount of drag in the paper test. I find I still need to crank down another 0.04-0.05mm from the automatically measured number to get a good first layer.
Question: During z-offset measurement the head lowers until the bed leveling sensor trips. The head then lowers until the nozzle touches the bed. I assume the measured z-offset is the delta between when the sensor trips and the nozzle touches, less a certain target nozzle to bed distance. Does anyone know what that target distance is? In other words, what is the printer choosing for the nozzle to bed distance?