repeatable vs accurate switch

I am working on a machine design that needs a signal that is repeatable to
a very tight tolerance. I don't care if it trips at 0.250" or at
0.000250", as long as it is the same each time. My thoughts now are
focusing on optical sensors. First is just a plane flag through an optical
coupler, and the second is a mechanical amplified version, IE a 10X gear
driven optical aperture.

Any pitfalls or ideas that come to mind??

Thanks Don