Law is not algebra. Concepts are not always well-defined, even important ones. “Fairness” is an issue that crops up constantly in criminal matters, and it too is poorly defined, if at all. The concept of reasonableness also pops up in issues such as reasonable necessity in self defence and in lots of other places. And the application of the concept to whether a prosecution case is proved, to whether a killing is justified by self-defence, and to whether a “reasonable man” would have mopped a spill off the floor so preventing an injury demonstrates that “reasonableness” is applied so widely that no over-arching definition which applies to all such situations yet is as precise as a proposition from Euclid can ever exist.
This is why threads which ask “If someone is coming through my front door in the dark of night, is it OK if I shoot him?” can’t be answered definitively as though someone were asking what is the length of the hypotenuse of some particular triangle. To judge reasonableness, you have to use your own common sense compass.
The law values a priori “knowability” of whether a person can determine if they are about to commit an offence in a theoretical sense, but not to the end of reckoning, or in truth, very far at all. The idea that a person should be able to know in advance of an action whether they are breaking the law tends to arise most in matters of the creation of statutory offences - it should in principle be possible for a person to know in advance whether arranging their affairs in some particular way will fall foul of the tax laws, for example. But individual facts of different cases in the general area of criminal and other branches of the law are too variable to give advisory answers up front, or definitions of of vague concepts, because there is no way to avoid one small fact changing everything.
Thus, the general answer to the question I posed above about a home invader might be yes in some particular jurisdiction, but that might change if the person coming through the door was unarmed and in a wheelchair, and to the knowledge of the shooter a family member of the household was known to be confined to a wheelchair.
This is why we get a dozen people to thrash this stuff out. The reasonableness of many if not most things can only be judged after the event.