Can I autopopulate a custom field based on another custom field ?

[Shivi Gupta]

I have a field named

"No.of users" ={0-50, 51-100,101-200}

I have another field called "User Points "

So ,if (No.of users=0-50 ) , User point = 0 ;

      if (No.of users=51-100), User point = 1 ;

and so on . 

 I have tried something on script runner but since I am not a good coder , I am not able to get through this . Can anyone help me with this ?