Change the bone's rotation driver to Euler.

In a Choreography, Action or Pose window:

• Select the bone.
• Create a channel for it by rotating it an arbitrary amount (probably around the Z axis).
• Right-click on "Rotate", which is under "Transform" in the Properties palette.
• Choose "Convert Driver To > Euler"
• Go to the frame where you want to bone to stop rotating.
• On the end frame, enter the total number of degrees you want the bone to rotate.