"How do you generate CI?"
The simple, approximate version of CI is the angle of a line between
- the point on the curve that is 0.1 density above FB+F, and
- the point on the curve with an exposure of 2.0 greater than the first point.
But this isn't exactly correct: the points are really on arcs of 0.2 and 2.2 scale units (x and y axis drawn to same scale) with a centre at the intersection of the horizontal FB+F line and the straight line intersecting the two points. Most easily found by using a little ruler with marks at 0.0, 0.2 and 2.2 units and placing it on the curve so that the 0.0 mark is on the FB+F line and the other two marks are on the curve; or in software by iteration based on the initial value given by the simple method.