A screw is usually measured from the tip of its shank to the base of its head. Maybe that is what you're doing; the 2mm mark is perfectly aligned with the base of the head.
You've provided a diagram and parts list for a K100RS 4-valve Brick. Your Brick's screw shank measures 85mm which is the correct shank length for the
crank side protection bar screw of the K100RS 4-V. Please verify the model and year of your Brick. If your Brick
is a K75C, consult the correct parts fiche diagram and list.
In the USA,
MaxBMW's parts fiche and Bob's BMW/Ducati show the screw shank length for the screws on both sides of a K75C as 65mm but their
head designs might differ because their part numbers differ.
Somebody who owns a K75 and has installed these contraptions—regardless of their effect on engine protection—should chime in.