Unanswered Auto level offset

Trial and error.

Usually you print a skirt or single layer object, then measure it with calipers, if it's off your offset will be the difference of how much it is off.

 

I usually do what Waldo suggested and print a single layer item and compare it to this:

https://printedsolid.com/blogs/news/tagged/first-layer

Adjust the Z offset (up if too close, down if too high) and reprint until you have a perfect number