I always like questions like this as the accuracy of the system in general is probably more important to know.
have a repeat accuracy of better than 0.001 mm
Is that really the manufactures specification? It is quite deceptive of them. Under what conditions would such a rating be true. That is about 4 hundred-thousandths of an inch. If you breath on it your likely to heat it up enough to change the reading not to mention how the reading will change as the machine itself heats and cools. A speck of dust in around 0.00002" so if the sensor or actuator is not perfectly clean then your reading will be an order of magnitude off (gasp only good to 4 ten-thousandths of an inch).
A typical mechanical switch will vary a bit in the exact position it opens and close due to several factors. The spring loaded parts inside will react slightly differently with temperature changes. The contacts also don't instantly change from open to closed (or closed to open) they 'bounce' a little so which of these bounces is Mach likely to see?
Think of the switch contacts as a resistor, when the contacts are closed you have close to zero ohms of resistance, when open the contacts have nearly infinite resistance. As the contacts start to open the resistance starts to rise every so slightly. Witch a good contact design this resistance change is fairly accurate and expensive contact probes look for this change is in resistance which provides a more repeatable indication that the switch is opening.
Back to the job that the OP is trying to do. You may find it is more accurate to move until you contact the switch and then back off of it really slowly.