how to set a max health and mana

kinda new to coding thought I did it right but guess not so whats currently happening is if I go below 0 will go to -10 then snap back to 0 but visually looks bad seeing -10 for a second. other problem is when the regen is occurring it regens to 101% instead of 100%. ive tried moving where I call the

if (currentMana > maxMana) currentMana = maxMana; in void update and void start and similar result.

using UnityEngine.UI;

public class DamageScript : MonoBehaviour { public float currentHealth = 100f; public float maxHealth = 100f; public float damage = 20f; public Image currentHealthBar; public Text HealthratioText; public Image currentManaBar; public Text ManaRatioText; public float currentMana = 100f; public float maxMana = 100f; public float minMana=0f; public float drainMana = 10f; private float t = 0.0f; private float threshold = 1.0f;

 void Start()
 {
     currentHealth = maxHealth;
     currentMana = maxMana;
     UpdateHealthbar ();
     UpdateManabar ();
 }
     public void UpdateManabar()
     {
         float ratio = currentMana / maxMana;
         currentManaBar.rectTransform.localScale = new Vector3 (ratio, 1, 1);
         ManaRatioText.text = (ratio * 100).ToString ("0") + '%';
     if (currentMana > maxMana)
         currentMana = maxMana;
     if (currentMana < minMana)
         currentMana = minMana;
     }

 public void UpdateHealthbar()
 {
     float ratio = currentHealth / maxHealth;
     currentHealthBar.rectTransform.localScale = new Vector3 (ratio, 1, 1);
     HealthratioText.text = (ratio * 100).ToString ("0") + '%';
 }
 void OnCollisionEnter (Collision col)
 {
     if (col.transform.tag == "Bullet") {
         currentHealth -= damage;
         print ("hurt" + currentHealth);
         if (currentHealth <= 0)
             Destroy (gameObject);
         UpdateHealthbar ();
     }

     else if (col.transform.tag == "Enemy") {
             currentHealth -= damage;    
             print ("hurt" + currentHealth);
         if (currentHealth <= 0)
             Destroy (gameObject);    
                 UpdateHealthbar ();
     }
         }

     void Update ()
 {
     t += Time.deltaTime;
     if (t >= threshold) {    
         t = 0.0f;
         currentMana++;
         UpdateManabar ();
     }
      if (Input.GetKeyDown (KeyCode.LeftShift))
         {
         currentMana -= drainMana;
             print ("manaDrain" + currentMana);
             UpdateManabar ();
         }
     }
 }