I see. You need to add a z probe and define it on the same pin as z min endstop since you use it as z min. If you do this, it will add z probe height to the current position which you then would set to 20.
Next trick would be to set homing order to one of the preheat version (with preheat temperature set to 0). Then you can also define the x,y position for z homing (so that is on top of the sensor). This will only work if tool is at least at 20mm height so you need to be either carefully to only home when z > 20 or raise tool 20mm before homing. That would work but if you are < 20mm from to away you hit top position and without endstop there it might also do bad things.
