How hard should I scratch my chinchilla??

Hi everyone! I've got an 8month old chinchilla male named Bubba. I've had him since he was 2months old and he's an amazing pet. I was wondering if anyone could explain how hard to scratch underneath his chin or other areas of his body. I'm a 25 year old male and while I'm not huge and muscular I still stay away from doing this because I'm afraid I'll hurt my little guy. I always pet him and he seems to love it because he makes a bubbling sound when I do. Then he grooms my hand after I do it every time haha. Thanks for any advice!!

, Jake