I found a good way to calibrate the nozzel (1) to top camera distance:

- Put a small paper label on a known PCB position.
- Pick up a small nozzel, then put the nozzel down to an rubber stamp ink pad
- Move the nozzel to the position (of a part) manually drive the nozzel down, until it touch the label.
- You will see a circle on the label.
- Move the camera to this position, it should be exact in the center of the circle
- Don´t forget to clean the nozzel :-)

best regards
Uwe