How do I create variable Jump Height for a platformer?

I am trying to put together a platformer style game but am unable to get variable jump height to work. I can jump, I can double jump, but all my jumps are the exact same height. There is no "short hop" version by pressing the jump button and letting go quickly.

I am using C#, the code I am using is below.

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(PlayerPhysics))]
 public class PlayerController : MonoBehaviour {
     
     //Player Handling
     public float gravity = 20;
     public float walkSpeed = 10.0f;
     public float runSpeed = 20.0f;
     public float acceleration = 30.0f;
     public float jumpHeight = 12.0f;
     public float jumpAirTime = 0f;
     public float jumpMaxAirTime = 0.2f;
     
     public float timeOffGround = 0f;
     private float currentSpeed;
     private float targetSpeed;
     private Vector2 amountToMove;
     
     //sloppy double jump stuff
     private bool djump = true; //This must be set to true or false to enable double jump; set to true on item pickup or the like for ingame toggle.
     public bool djumpused = true; //checks whether or not the jump has been used; set to true to any spawn off the ground
                                   //will not result in the ability to double jump
         
     private PlayerPhysics playerPhysics;
     
     void Start () 
     {
     playerPhysics = GetComponent<PlayerPhysics>();
     }
     
     void Update () 
     {        
         // Reset acceleration upon collision
         if (playerPhysics.movementStopped)
         {
             targetSpeed = 0;
             currentSpeed = 0;
         }
         
         
         if (playerPhysics.grounded)
         {
             amountToMove.y = 0;
             timeOffGround = 0;
             djumpused = false;
             
             
             //Jump
             if (Input.GetKeyDown(KeyCode.Space)) // If jump is pressed
             {
                 jumpAirTime = jumpMaxAirTime; //Start jump
                 jumpAirTime -= Time.deltaTime; //Timer decreases
                 
             }
         
         
         if (amountToMove.y == 0 && Input.GetKeyDown (KeyCode.Space) && jumpAirTime > 0) //If you are jumping
         {
             amountToMove.y = jumpHeight; //jumping force!
             jumpAirTime -= Time.deltaTime; //Timer decreases
         }
         
              else
                 {
                     amountToMove.y = 0;
                     jumpAirTime = 0;
                     timeOffGround =0;
                 }
         
                 
         }
         
         if (Input.GetKeyDown (KeyCode.Space) && djumpused == false && !playerPhysics.grounded)
             {
                 amountToMove.y = jumpHeight;
                 djumpused = true;
                 
             }
         
         else //gravity takes over at this point
         {
             timeOffGround += Time.deltaTime;
         }
         
         
         
         
         
         //Input
         float speed = (Input.GetButton ("Run"))?runSpeed:walkSpeed;
     targetSpeed = Input.GetAxisRaw("Horizontal") * speed;
     currentSpeed = IncrementTowards(currentSpeed, targetSpeed,acceleration);
     
     
         
         amountToMove.x = currentSpeed;
         amountToMove.y -= gravity * Time.deltaTime;
         playerPhysics.Move(amountToMove*Time.deltaTime);
         
     }
             
     // Increase n towards target by speed
     private float IncrementTowards(float n, float target, float a)
     {
         if (n == target)
         {
             return n;
         }
         else
         {
             float dir = Mathf.Sign (target - n); //must n be increased or decreainsed to get closer to target
             n += a * Time.deltaTime * dir;
             return (dir == Mathf.Sign (target-n))? n: target; //if n has increased over target then return target, otherwise return n. The ? n: is a shorthand if statement.
             
     
         }
     }
 
 }