50 mm should allow you to machine a plastic block with a glued on metal top (maybe stainless steel) . Once you make that surface cut the top metal and measure it's height, could also be a bolt screwed and glued into the top and machined flat.
Connect a battery with dropping resistor to a LED and connect that to the tip somehow, connect the insulated block metal top to the other end of the battery.
Bring the tip down when it lights you know your height from the base/table.
An issue would be practicing to not destroy the tip.
This would be something like a electronic center finder.
Maybe you can use a plastic piece of known thickness that has both sides parallel and use an optic microscope focused at the landing point of the tip on the plastic to see when the tip touches. The plastic might be soft enough not to break the tip
