My sense at the moment is that there may be sufficient discretion within the concept of what is a necessary and proportionate response, because proportionality is not a precise concept either. Self-defence cases are replete with observations by judges to the effect that you can't be expected to weigh exactly how much force you need to use in response to a serious threat to your life or your person. So I'm wondering what this would add to those other regions of discretion.
On February 14th, 2012. See this statement in context.