> Out of interest, why should it be anything other than over 50%?
England and Scotland have been in a binding union since 1707. Suppose that one day, many years from now obviously, Scotland has a vote and 52% of 72% of the voting population decide to end that union, persuaded by a charismatic leader in Scotland at that instant in time, while a buffoon led England at that same instant. That is hardly a stable way to decide a "forever policy" - it's not like you can undo it four years later if you realise it was an error and that the charismatic leader was indeed very charismatic, but wrong.
So for a long term change, you'd probably look for something better than 52% of 72%, especially when the known reality of the status quo can never be sold in quite such an exciting way by a dull and boring leader as the sunlit uplands of the dream by a charismatic rebel.