Custom Normals 3: "Two Halves" Face

Select your head/face mesh and open the right-side panel (press N). Under the Abnormal tab, there is a drop down selector that should show a list of your mesh's vertex groups. Select the "Custom Normals" vertex group we created earlier.

Start Abnormal with the "Start Normal Editor" button. Every vertex should already be selected, and you'll see long pink lines in every direction representing the directions of the normals. It's kind of hard to see what we're doing, so this time let's reduce the Normals Length (right-side panel) down to a reasonable size, like 0.03-0.04.

Under "Align to Axis", press the -Y button. The shading of your face should now appear completely flat. We're going to select one half of the face, but to do that we need a good view of it. Force an orthographic front view by clicking the corresponding axis in the top-right corner (-Y). Now deselect everything (Alt+A) and then Box Select (B) one half of the face. Don't select the center column of vertices. Enable Mirror Axis on the X axis. With one half of the face is selected, use the gizmo to rotate that side of the face away on the Z axis (blue).

The face should now have two flat shaded halves. Disable Mirror Axis. The crease in the middle is a little sharp, but we can simply select everything (A) and apply Smooth Selected Normals with a desired Smooth Strength and Smooth Iterations. A strength of 1.0 with the default 5 iterations is usually enough.