How to align Axis parallel to horizon (X, Z plane) for rotating using AngleAxis method

How do I calculate the Axis on the spine bones so that when I rotate using AngleAxis(), it Pitchs’ along the horizon without messing along the roll?

I asked this question 2 years ago. I thought I might as well post an answer. So there you go.

Its done with the help of a helper transform placed perpendicular to the Axes plane I’m planning to AngleAxis() along.

That’s all.