I think you should specify the northern border of the contiguous states. Otherwise, I bet 99% of Canada’s population lives south of Alaska northern border.
I hesitated to be more specific, but figured (probably wrongly so, should always be explicit when stating things like this) that it's implicitly obvious it's about the ~49th parallel border. In fact you can draw the line well below that border[0].
Another way to put it is that half of Canadian's population lives ~between Toronto and Montréal.
