Going by memory, I'd say something like this:
const FingerList fingers = hand.fingers();
for (int f = 0; f < 5; f++) {
Finger finger = fingers[f];
float angleInDegrees = RAD_TO_DEG*finger.bone(Bone::TYPE_PROXIMAL).direction().angleTo(finger.bone(Bone::TYPE_INTERMEDIATE).direction());
// similarly for other phalanx pairs
}
...or your question is more subtle?