There is some wiggle room, maybe .4 mm. The illuminating LED is about 45mm away, so a .4 mm movement causes roughly a 1% change in shadow size (45-.4)/45, so +.03mm deviation. One way to reduce this is to move the LED even further away, or have some sort of spring-loaded roller that holds the filament in place. In practice I have not seen much variation when I wiggle the filament in the sensor. I wonder if moving the filament away from the sensor also diffuses the shadow edges, thereby effectively compensating for an increase in shadow size.