InverseSignedAngleBetweenXZ.cs

/// <summary>
/// Returns the signed angle between two transforms, from the perspective of the From transform, ignoring the Y axis
/// </summary>
/// <param name="fromTransform"></param>
/// <param name="toTransform"></param>
/// <returns></returns>
public static float InverseSignedAngleBetweenXZ(this Transform fromTransform, Transform toTransform)
{
    // Remove the Y coordinate from the reference position and the target position
    var toPosition = new Vector3(toTransform.position.x, 0, toTransform.position.z);
    var fromPosition = new Vector3(fromTransform.position.x, 0, fromTransform.position.z);

    // Get the direction to the target position, using the From position as the reference position
    var directionToTarget = fromTransform.InverseTransformDirection(toPosition - fromPosition);
    var localForward = fromTransform.InverseTransformDirection(fromTransform.forward);

    return Vector3.SignedAngle(localForward, directionToTarget, fromTransform.up);
}