Changing quality setting bases on FPS ?

Well, the Quality Settings provide functionality for this! It's a really simple script, and I've already written it. Here you go!

 public class QualityAutomater : MonoBehaviour {

     // The min and max frameRate determines at what point it will switch up or down
     public double maxFrameRate;
     public double minFrameRate;

     // Sometimes, there are lag spikes that happen over one or two frames,
     // but are not necessarily related to the quality settings. The 
     // switchThreshold is the number of frames in a row that must be too
     // slow or fast before a switch occurs.
     public int switchThreshold;
     
     int upCounter;
     int downCounter;
     
     void Update () {
         if((1 / Time.smoothDeltaTime) > maxFrameRate)
         {
             upCounter++;
             if(upCounter > switchThreshold)
             {
                 QualitySettings.IncreaseLevel();
                 upCounter = -3;
             }
         } else {
             upCounter = 0;
         }
         //Debug.Log(upCounter);
         
         if((1 / Time.smoothDeltaTime) < minFrameRate)
         {
             downCounter++;
             if(downCounter > switchThreshold)
             {
                 QualitySettings.DecreaseLevel();
                 downCounter = -3;
             }
         } else {
             downCounter = 0;
         }
     }
 }