Interesting question. The answer is half scale deflection, but I can't actually find that written down anywhere. The closest thing that I can find (in Australian documents)is in CAO 40.2.1 section 3.5, tolerances for approaches during an instrument rating test. Can anybody do better?
One common sense point here. When flying any approach, there should be no SUSTAINED errors in excess of the permitted tolerance. If you go outside the permitted tolerance, briefly and then smoothly and using good technique bring the aircraft back within tolerance again, no problem. A sustained excursion beyond tolerance or even uncontrolled tracking within tolerance should be followed immediately by a missed approach and another go.