It turns out old-school plotter pen nibs skid right off the rounded top of the switch lever:
You can’t see the nib inside the cap, but you get the idea.
Flattening the top and adding a snippet of masking tape produces a better outcome:
I aligned the flat section so it’s parallel to the platform when the switch activates.
Stipulated: plotter pens aren’t a good test for tool length probing, because they have a locating flange to ensure a consistent position in the pen holder and a rigidly controlled flange-to-tip length:
What’s going on here involves configuring and testing bCNC’s overall tool change process: not using cutting tools preserves both sanity and hardware!