But wait. Here's contrast index values from the Xtol information data sheet that use different values.

Kodak Data sheet excerpt - processing aims.jpg

The data sheet vs Kodak's CI/NDR Chart

N - 0.58, 0.58
+1 - 0.65, 0.70
+2 - 0.75, 0.88
+3 - 0.85, 1.17

Why the difference? One is for adjusting for different luminance ranges and one is for adjusting for under exposure. In other words, pushing for contrast vs pushing for speed. For pushing for speed, the scene luminance range remains constant, but the exposure is shifted to the left. This lowers the negative density range as the shadows drop further down into the film's toe. Increasing processing increases the the contrast of the film making up for the loss from under exposure. It also increases density in the toe. This effectively increases the film speed. The general rule of thumb is a stop increase in contrast increases the "speed" 1/3 stop. A film under exposed one stop and push processed results in only a 2/3 stop under exposure. In order to match the same density range as a normally exposed and processed film, the under exposed film is processed 2/3 of a stop contrast for each 1 stop under exposure.

The difference between pushing for contrast and pushing for speed is that the difference between the steps for pushing for contrast is 0.30, while the difference between pushing for speed is 0.20.

Contrast Indexes for pushing for contrast vs pushing for speed

N
1.05 / (2.20 - 0.40) = 0.58
1.05 / (2.20 - 0.40) = 0.58

N+1
1.05 / (1.9 - 0.40) = 0.70
1.05 / (2.0 - 0.40) = 0.656

N+2
1.05 / (1.60 - 0.40) = 0.88
1.05 / (1.80 - 0.40) = 0.75

N+3
1.05 / (1.30 - 0.40) = 1.17
1.05 / (1.60 - 0.40) = 0.875

All the answers are out there. They're just slightly hidden in plain sight.