"About percentages at a referendum, the cleanest approach would be that at least 50% of the total electorate would have to have chosen for one of the two options. That way you could actually state that the will of the people is X. It would be quite hard for either option to reach that point, though."
Actually the cleanest approach is to have a substantial, say 2/3 majority for a change in the status quo. That means you don't go casting back and forth (even assuming you can and in this case we probably can't - once we're out we're out) on small sways of opinion or that you commit future generations to a potentially bad decision on the basis of a small majority.