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

ELEGOO

ELEGOO NEPTUNE

No

No

0.2

100

Takes around 30 minutes on my Neptune 4

The extension push fits over the two poles of the main lever support - a little filing of the holes was needed - a bit of glue to keep it firm. The pivot arm is now held in place by a copper wire through the two posts. A weight or magnet could be used on the extension to hold everything flat to the bed.

As you can see from the pic the device can now operate near the front of the bed - will likely need a weight on the other end to get right up to the front without wobble.
Also possibly needs a notch in the back part to allow more clearance for the nozzle.
However it works nicely - 0.1mm Z movement causes about 4mm movement of the needle pointer. So 0.01 mm movement should give 0.4mm movement of the needle.
A piece of white card stuck to the needle end post will help in marking the position.
Notched extension added.
I have added an alternative pivot post which extends sideways instead of back - this should allow the device to be used on smaller build plates - weights can be added to the rectangular sides to hold it steady.
Might require some sideways and some backward extension to be usable. See back and Sides.stl - it looks like it is starting to turn into a Klingon Bird of Prey

This version allows weights or magnets to be put on the sides to hold it flat.
Two 20x3mm neodymium magnets work nicely with a PEI spring steel plate.
Works OK for bed level corner checking but not Z offset setting - that is still done with a bit of paper