Frank,

It's a case of input to output. The average scene has a log subject luminance range of 2.2 or 7 1/3 stops. With flare, it is about 1.80 to 1.90. If the resulting density range of the negative (DR) equaled the scene's luminance range, the negative DR would be about 1.80, but it's not. The aim DR for a diffusion enlarger is around 1.05 or just over 1/2 the input. That would be a slope of around 0.58 or a Contrast Index of 0.58. The equation to calculate the slope of a line is rise / run.

In order to have the printed image approximate the range of the original subject, the paper must have around twice the slope of it's curve. The basic equation is Slope Neg x slope paper = 1.00. A value of one, or unity, is an approximate 1:1 relationship between the original subject and the resulting print.

If you are curious to read more on this subject, it's known as Tone Reproduction Theory.