I'm thinking that the hole in your disc should be much larger than the aperture on the data sheet. Since you are moving the disc with a stepper motor, how are you going to align the stopping place of the disc (due to the motor steps) such that the hole is aligned with the optical axis of the photointerupter? If you make the hole too small, that will be very difficult. The motor likely has 200 steps per revolution. Figure the arc length between motor steps at the radius where the hole is to be drilled, and make the hole diameter be about 3/4 of one step...