"I've always felt that the legal term 'reasonable' is open to interpretation by the person doing the judging"

Yes, that is a good point. However, it does give flexibility in an assessment in differing situations. What is defined as "fair" in one point in time, in a multitude of situations, or in differing cultures could vary significantly and could be very subjective.

Years ago, I heard of situations (accurate, or not) where families in the extreme north were faced with starvation with the lack of food. Cultural norms were to sacrifice the weakest family members firs, the old a t young. This seemed reaonable, based on survival logic in that culture. However, in our culture this approachb may not seem to be fair - women and children tend to be seen as more important when it comes to survival strategies (additionally, it may also seem unfair to those being sacrificed).