I started a large format class this semester a couple of weeks ago. We have started learning the zone system. The instructor has told us that N+1 is 150%, N+2 is 300%, N-1 is 75% and N-2 is 50% of overall development time. I am curious as to what others think about this. I have been using the zone system for over a year now with 120 film and multiple backs. Everything I have read says that adding 15%-20% is equivalent to N+1. This is what I have been using for the past year. Who is correct? Having I been doing it wrong this whole time. Will doing what he says actually increase/ decease the highlights by a full zone.