Once you see it print correctly, you will know what you are looking for. But probably the right place to start is set the #define Z_PROBE_OFFSET_FROM_EXTRUDER value a little bit larger (less negative) than you need. Maybe by 1mm. This will cause it to air print. Depending on how high you are up above the glass, adjust that number so you have half of the air space between the nozzle and the bed. Print again. See if it still air prints. Keep adjusting it a little at a time until the plastic starts to get squished into the glass.

You do need a small gap between the nozzle and the glass. But it is very difficult to measure and get right. It is much easier to adjust the #define Z_PROBE_OFFSET_FROM_EXTRUDER number until the plastic is getting pressed into the glass the right amount.