Leap Motion - Angle of proximal bone to metacarpal

I am trying to get the angle between the bones, such as the metacarpal bone and the proximal bone (angle of moving the finger side to side, for example the angle when your index finger is as close to your thumb as you can move it and then the angle when your index finger is as close to your middle finger as you can move it).

I have tried using Vector3.Angle with the direction of the bones but that doesn't work as it includes the bending of the finger, so if the hand is in a fist it gives a completely different value to an open hand.

What i really want is a way i can "normalize" (i know normalizing isn't the correct term but it's the best i could think of) the direction of the bones so that even if the finger is bent, the direction vector would still point out forwards and not down, but would be in the direction of the finger (side to side).

I have added a diagram below to try and illustrate what i mean.

In the second diagram, the blue represents what i currently get if i use the bone's directions, the green is the metacarpal direction and the red is what i want (from the side view). The first diagram shows what i am looking for from a top-down view. The blue line is the metacarpal bone direction and in this example the red line is the proximal bone direction, with the green smudge representing the angle i am looking for.

To get this value, you need to "uncurl" the finger direction based on the current metacarpal direction. It's a little involved in the end; you have to construct some basis vectors in order to uncurl the hand along juuust the right axis. Hopefully the comments in this example script will explain everything.

using Leap;
using Leap.Unity;
using UnityEngine;

public class MeasureIndexSplay : MonoBehaviour {

  // Update is called once per frame
  void Update () {
    var hand = Hands.Get(Chirality.Right);
    if (hand != null) {
      Debug.Log(GetIndexSplayAngle(hand));
    }
  }

  // Some member variables for drawing gizmos.
  private Ray _metacarpalRay;
  private Ray _proximalRay;
  private Ray _uncurledRay;

  /// <summary>
  /// This method returns the angle of the proximal bone of the index finger relative to
  /// its metacarpal, when ignoring any angle due to the curling of the finger.
  /// 
  /// In other words, this method measures the "side-to-side" angle of the finger.
  /// </summary>
  public float GetIndexSplayAngle(Hand h) {
    var index = h.GetIndex();

    // These are the directions we care about.
    var metacarpalDir = index.bones[0].Direction.ToVector3();
    var proximalDir   = index.bones[1].Direction.ToVector3();

    // Let's start with the palm basis vectors.
    var distalAxis = h.DistalAxis(); // finger axis
    var radialAxis = h.RadialAxis(); // thumb axis
    var palmarAxis = h.PalmarAxis(); // palm axis

    // We need a basis whose forward direction is aligned to the metacarpal, so we can
    // uncurl the finger with the proper uncurling axis. The hand's palm basis is close,
    // but not aligned with any particular finger, so let's fix that.
    //
    // We construct a rotation from the palm "finger axis" to align it to the metacarpal
    // direction. Then we apply that same rotation to the other two basis vectors so
    // that we still have a set of orthogonal basis vectors.
    var metacarpalRotation = Quaternion.FromToRotation(distalAxis, metacarpalDir);
    distalAxis = metacarpalRotation * distalAxis;
    radialAxis = metacarpalRotation * radialAxis;
    palmarAxis = metacarpalRotation * palmarAxis;

    // Note: At this point, we don't actually need the distal axis anymore, and we
    // don't need to use the palmar axis, either. They're included above to clarify that
    // we're able to apply the aligning rotation to each axis to maintain a set of
    // orthogonal basis vectors, in case we wanted a complete "metacarpal-aligned basis"
    // for performing other calculations.

    // The radial axis, which has now been rotated a bit to be orthogonal to our
    // metacarpal, is the axis pointing generally towards the thumb. This is our curl
    // axis.
    // If you're unfamiliar with using directions as rotation axes, check out the images
    // here: https://en.wikipedia.org/wiki/Right-hand_rule
    var curlAxis = radialAxis;

    // We want to "uncurl" the proximal bone so that it is in line with the metacarpal,
    // when considered only on the radial plane -- this is the plane defined by the
    // direction approximately towards the thumb, and after the above step, it's also
    // orthogonal to the direction our metacarpal is facing.
    var proximalOnRadialPlane = Vector3.ProjectOnPlane(proximalDir, radialAxis);
    var curlAngle = Vector3.SignedAngle(metacarpalDir, proximalOnRadialPlane,
                                        curlAxis);

    // Construct the uncurling rotation from the axis and angle and apply it to the
    // *original* bone direction. We determined the angle of positive curl, so our
    // rotation flips its sign to rotate the other direction -- to _un_curl.
    var uncurlingRotation = Quaternion.AngleAxis(-curlAngle, curlAxis);
    var uncurledProximal = uncurlingRotation * proximalDir;

    // Upload some data for gizmo drawing (optional).
    _metacarpalRay = new Ray(index.bones[0].PrevJoint.ToVector3(),
                             index.bones[0].Direction.ToVector3());
    _proximalRay = new Ray(index.bones[1].PrevJoint.ToVector3(),
                           index.bones[1].Direction.ToVector3());
    _uncurledRay = new Ray(index.bones[1].PrevJoint.ToVector3(),
                           uncurledProximal);

    // This final direction is now uncurled and can be compared against the direction
    // of the metacarpal under the assumption it was constructed from an open hand.
    return Vector3.Angle(metacarpalDir, uncurledProximal);
  }

  // Draw some gizmos for debugging purposes.
  public void OnDrawGizmos() {
    Gizmos.color = Color.white;
    Gizmos.DrawRay(_metacarpalRay.origin, _metacarpalRay.direction);

    Gizmos.color = Color.blue;
    Gizmos.DrawRay(_proximalRay.origin, _proximalRay.direction);

    Gizmos.color = Color.red;
    Gizmos.DrawRay(_uncurledRay.origin, _uncurledRay.direction);
  }
}

For what it's worth, while the index finger is curled, tracked Leap hands don't have a whole lot of flexibility on this axis.