How to calculate pelvis position from VR head?

We are developing a game with full body IK. We really want to know how to calculate the pelvis position from the VR head, so that when player head rotate down, they cant see the body inside. Here is some methos that we tried but not success.

  1. Offset the camera view with AdditiveOffset function: its wrong because the rotate point and view point are not the same, so player will feel motion sickness.
  2. Add an arrow component as child of camera and use its position as pelvis position. Its only good with moving the head, but its extremely bad with rotating the head.
  3. Asign the position of the VR head directly to pelvis position. Its very bad with rotating the head.
  4. Create a variable call player pivot position. Then calculate the position of the pelvis base on the distance from head to pivot position. But its very hard code and we cant predict the movement of player.
  5. Calculate camera velocity each frame, then use it to add to the pivot position. But then, it only good with moving, not rotating.
    All the position above, we only care about the position X and Y, as the Z is easy to calculate.
    We want to do exactly the same as the VR game bonework. But now this is the only problem that we have. Can anyone turn us to the right direction?