You got good answers, 0.3 is for the x-axis. The y-axis is your result and it may vary...
Originally Posted by lhalcong
Think of what you are trying to achieve, this may make sense to you:
For example, to "raise Zone VIII to IX" is one way of defining N+1. You basically say you like what you get when Zone IX is developed to N. "But this time, I don't have any Zone IX, my subject only goes up to Zone VIII"... "So I'll develop longer, until Zone VIII hits the same density I usually get for Zone IX."
So look at the N curve, the density at Zone IX. Then look across left and right in 0.3 increments on the x-axis. Look for longer development time where Zone VIII hits the same density (That will become the N+1 time), and what shorter development time brings Zone X down to that density (That will become the N-1 time). And so on. If your curves don't hit the line, you can extrapolate the time.
FYI Minor White instructed students to draw a "heavy horizontal line" across where N meets Zone VIII.