I have a "feeling" that the oft-quoted term "reasonable belief" is not worth the time it takes to type it unless you are very knowledgeable of the society in which your trial might occur.
I could take you into communities within Tennessee where where the act of merely pushing someone in an angry manner will substantiate the existence of the "reasonable belief" of danger of serious injury or death. On the other hand, I suspect that there are other areas of our country where the use of a gun to fend off an knife attack would be counsidered inappropriate because of an escalation of force.
It is a good thing to be aware of the language of the law. It is not a good thing to assume that it replaces your own judgement when it comes to your self-protection. Hopefully, knowledge and judgement combined will keep you out of both jail and the hospital (or morgue!).