z_start might be a better option for reference. Tecan training typically suggests that z_start is set to “right above” the top of the well. In practice, usually 0-1mm above the well. This height is used as a starting point when trying to detect liquid, so it is important that it never be set to a height within the well where liquid could be present.
z_dispense is usually set at a small distance below the well opening to prevent well contamination due to sputtering with poorly optimized LCs. It could be any height between z_start to z_max depending on who framed the labware.