For 'N-1' read 'contract by one zone'. Similarly, 'N+2' means 'expand by one zone'. Roughly speaking, developing for N-1 means reducing the contrast of the film (by reducing development) so the effect on the film would be as if the scene had one zone less contrast.(posted 9217 days ago)In your example, the scene has a contrast range of 10 zones (14.5- 4.5=10). If your film can handle 10 stops, just develop as N. If the film can normally only handle 9 stops, develop as N-1. Etc.
For a real explanation, see Adams 'The Negative'.