Tracking Forward HTC Vive Movement

Hi! I'm trying to use an HTC Vive to track a player's forward movement inside a game. Right now the code I have works in a static x-direction, where your positive x movement would play a "walking forward" animation, negative x movement would play a "walking backward" animation, and a general "idle" trigger for when the hmd doesn't detect significant movement. Is there a way to track the "forward" direction the player will walk in, as opposed to hard coding a single x-direction?

Here's what I have so far:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class AnimatedController : MonoBehaviour
 {
     //how often we want the function to be called
     [SerializeField] float refresh_rate = 0.5f;
     //where we place the "camera" object under Camera Rig for SteamVR
     [SerializeField] Transform HMD;
     //[SerializeField] Transform build;
     [SerializeField] float error_margin = 0.3f;
     [SerializeField] float out_of_bounds = 1.0f;
     float xDir, yDir, zDir, xTemp, yTemp, zTemp;
 
     // Start is called before the first frame update
     void Start()
     {
         //after 0.1f seconds, calls the function TrackMovement every refresh_rate value
         InvokeRepeating("TrackMovement", .01f, refresh_rate);
         //retrieve the values of x, y, and z for the HMD
         xDir = HMD.transform.position.x;
         yDir = HMD.transform.position.y;
         zDir = HMD.transform.position.z;
     }
 
     void TrackMovement()
     {
         //retrieve values for comparison
         xTemp = HMD.transform.position.x;
         yTemp = HMD.transform.position.y;
         zTemp = HMD.transform.position.z;
 
         //If the refreshed value is greater than the current + idle margin error - forward
         if(xTemp > xDir + error_margin)
         {
             Debug.Log("Trigger Forward Walking Animation");
         }
         //If the refreshed value is less than the current - idle margin error - backward
         if (xTemp < xDir - error_margin)
         {
             Debug.Log("Trigger Backward Walking Animation");
         }
         //if the refreshed value is inbetween  idle margin errors - idle
         if (xTemp <= xDir + error_margin && xTemp >= xDir - error_margin)
         {
             Debug.Log("Trigger Idle Animation");
         }
         //if the refreshed value is out of bounds - falling
         if (zTemp > zDir + out_of_bounds || zTemp < zDir - out_of_bounds)
         {
             Debug.Log("Trigger Falling Animation");
         }
 
         xDir = xTemp;
         yDir = yTemp;
         zDir = zTemp;
     }
 }