I have a piece of torus face (from a more complex body) to analyse like this:

I would like to calculate the neutral axis length with:

• The surface TorusParams (TorusParams Property (ISurface)​) which give me:
  • a center point (x, y, z)
  • an axis vector (x, y, z)
  • a major radius (R on image)
  • a minor radius (r on image)
• The extremity curves (CircleParams Property (ICurve)) which give me:
  • the extremity points of neutral axis (x, y, z)
  • other informations...

To calculate this length:

• I calculate an angle (rad) but the angle can be:
  • small: angle
  • large: 2*PI - angle
• To determine the angle to use, I calculate:
  • the [center, extremity1] vector (x, y, z)
  • the [center, extremity2] vector (x, y, z)
  • the cross product of the vectors (x, y, z)
  • the dot product with the torus axis vector
    • if the dot product is positive (same direction):
      • the torus turns in a clockwise direction
    • if the dot product is negative (opposite direction)
      • the torus turns in an anti-clockwise direction

My problem is:

• The torus axis is based on a starting and ending point (extremity1 or extremity2) but I don't know which one.

How can I find it?

Thank you for your help.