"Reasonable" is open for subjective judgment, which is precisely the purpose of a jury. It would be nice if it could be more specific, but that would create unreasonable loopholes for murder, and any specificity you might write into the law, would be up to a DA and jury to apply validity to.
Did the man break into your home with the knife or was he invited in, shot, and a knife placed in his hand?
Great in theory. In reality, the victims/suspects are put through hell and lawyers get rich.