PhoneRotation.cs

/// <summary>
/// This rotates the camera to match the orientation of the phone
/// </summary>
public void TiltPhone()
{
    // Helps to stop us getting caught in a bad change of states. Resets in ResetRotation().
    if (!tilting)
    {
        // Save our current rotation before doing anything else. This is where we'll return later.
        _resetRotation = transform.localRotation;
        // This is the opposite of the phones rotation when entering the tilt mode. 
        // We are aiming to negate by this value later.
        _negatePhoneRotation = Quaternion.Inverse(DeviceRotation.GetRotation());
        tilting = true;
        //debugText.enabled = true;
    }
    
    // None! This is 1 rotation offest by another. No idea how it works.
    // Why do you offset the right by the left? Who knows. It's magic.
    _desiredRotation = _negatePhoneRotation * DeviceRotation.GetRotation();

    // Set rotation at the end, assumes _desiredRotation has been set in 1 of the above if statements.
    transform.localRotation = _desiredRotation;
    // Cache it back into the conveniently shorter variable name.
    _currentRotation = transform.localRotation;
    //debugText.text = _desiredRotation.ToString();
}

public void ResetRotation()
{
    // Include anything special that needs to be reset separate to the coroutine.
    tilting = false;
    //debugText.enabled = false;

    StartCoroutine("LerpCameraBack");
}

// This works perfectly in all my test so far.
private IEnumerator LerpCameraBack()
{
    // How do I find the return angle and rotate back to normal? So scared.
    // Quaternions are the devil
    _currentRotation = transform.localRotation;
    // We will return to this point. desiredRotaiton doesn't change throughout the whole coroutine.
    _desiredRotation = _resetRotation;
    float lerpCompletion = 0;
    
    while (lerpCompletion < 0.99) 
    {
        lerpCompletion += Time.smoothDeltaTime * lerpMultiplier;
        var curveValue = curve.Evaluate(lerpCompletion);

        transform.localRotation = Quaternion.Slerp(_currentRotation, _desiredRotation, curveValue);

        yield return null;
    }

    // The reset rotation has now finished, set any states and variables that need setting.

    // Sets the rotation back, compensate for any remaining angle changes.
    transform.localRotation = _desiredRotation;
}