Better to put the mirror (or at least a reflective surface) on the outside edge of the disc. It has two advantages:
1. The edge is normally accessible, the middle of the disc is usually obscured by the counting mechanism and may have to pass through a small gap in the motor magnets.
2. The disc has to be kept in balance, it's easier to achieve that on the edge. If necessary, fit two reflectors, on opposite sides to maintain balance and divide the pulses by two.
My meters here have a black mark on the edge of the disc so it's easier to see if it is rotating, if yours is the same, use a reflective opto-switch and look for the gap in the reflection as the black section passes by rather then create a new one.
Brian.