That's really just to make sure your prints stick properly to the bed, honestly. I found that the best way to tune it is by tuning the z-offset (just hold the button during the first layer) while printing a large-ish cube, with the fill angle set horizontally (not in front of a computer at the moment to give you more specifics). It basically is a larger version of what happens at the end of the built-in z-offset calibration.