ADF has errors of up to +/- 5 degrees thus 5 degrees off indicated could actually be 10 degrees off. Safety lane as you call it (surveyed approach area) is +/- 10 degrees. Thus greater than +/- 5 could put you outside of the safe area. Trouble is, could also put you more or less on centre line! Don’t you just love an NDB approach.
By the way, I once had to fly an asymmetric NDB approach to minimums for real. Deep joy!