This class in conjunction with the Adaptive Quality class implements a basic adaptive quality mechanism in Unity.

AdaptiveViewport.cs

using System.Collections.Generic;
using UnityEngine;

/// <summary>
/// This class in conjunction with the Adaptive Quality class implements a basic adaptive quality mechanism in Unity.
/// Must be attached to the active camera in your scene.
/// </summary>
/// 
public class AdaptiveViewport : MonoBehaviour {

    [SerializeField]
    private int fullSizeQualityLevel = 5;
    [SerializeField]
    private int minSizeQualityLevel = 0;
    [SerializeField]
    private float minViewportSize = 0.5f;

    [SerializeField]
    private AdaptiveQuality qualityController;

    public float currentScale { get; private set; }

    private void OnEnable()
    {
        currentScale = 1.0f;

        if (qualityController)
        {
            qualityController.QualityChanged += QualityChangedEvent;
            SetScaleFromQuality(qualityController.qualityLevel);
        }
    }

    private void onDisable()
    {
        qualityController.QualityChanged -= QualityChangedEvent;
        UnityEngine.XR.XRSettings.renderViewportScale = 1.0f;
    }

    private void OnPreCull()
    {
        UnityEngine.XR.XRSettings.renderViewportScale = currentScale;
        QualitySettings.SetQualityLevel(qualityController.qualityLevel, false);
    }

    private void QualityChangedEvent(int newQuality, int previousQuality)
    {
        SetScaleFromQuality(newQuality);
    }

    private void SetScaleFromQuality(int quality)
    {
        int clampedQuality = Mathf.Clamp(quality, minSizeQualityLevel, fullSizeQualityLevel);

        float lerpVal = Mathf.InverseLerp(minSizeQualityLevel, fullSizeQualityLevel, clampedQuality);
        currentScale = Mathf.Lerp(minViewportSize, 1.0f, lerpVal);
    }
}