Using a bit of paper requires you 'feel' the resistance - This level tool allows you to have a visual indication. Based on my Neptune 4 when the nozzle is close to the bed, there is a 32mm gap between bottom edge of the print head frame and the build plate.
Using an eccentric pivot allows a tiny movement to be greatly magnified. A mark can be made on the pointer end of the device - in power off mode and manually moving the X and Y , the four corners can be checked for equal distance from the print head (nozzle)
As it stands now the needle part is not well supported - could do with a twin support at the pivot so the needle can be supported by a through pin. Needs work
There is now a pivot post with two support posts - I have not yet tried this version.

I might try to set it up with a laser diode so it puts a spot of light on the opposite wall of the room, giving 2m instead of 200mm indicator - not sure how practical that would be.

The concept is something I like - however when I tried it out, it is not possible to use it when the print head is at the front of the bed - I will need to do a rethink.
Added an extra bit to glue on to the main pointer base to extend it back - hopefully this will allow the head height to be measured when at the front of the bed