Hi oem_odm,
The word is overshoot.
The 6v DC motor is geared down to run a shaft for some purpose which involves the angular positioning of the shaft.
You wish to stop the rotation of the motor and shaft when something or the other lines up relative to the shaft.
Have you tried short circuiting the motor in order to see how much angular overshoot occurs ?
Have you tried repositioning the angular position of the sensor,
so that when it operates and shorts the motor,
the shaft will then stop in the appropriate place?
I am guessing that this motor is on a worm.
I think you will need a mechanical brake assembly attached to the motor,
to operate in conjunction with cutting the feed to the motor.
And possibly a small speed-relative small spring loaded part to adjust the photo sensing arrangement a few degrees to take into account overshoot at low speeds,
before the motor is up to running speed.
Unless your arrangement is such that the shaft is never required to stop without the motor reaching running speed. (thats why i put possibly).
Such a small spring loaded part need only be a little piece that has a hole for the light to go through, pivots a little against a spring, and alters with motor speed.
If such a part is required, don't forget that when the motor is stopped, it will return to rest position.
This may cause the light to be interrupted, so have the circuit such that this won't make the motor start again.
Please post more details on the assembly. Pictures if possible.
Cheers, John