Another guestimate method is using a known Canadian production figure multiplied by 10 (being Canada's always running an approximate 10% of US population).
Using this method can only be a guide, there's certainly exceptions and it's never going to be an accurate # either way but in cases where Canadian and US #'s are both known the 10% thing has often shown to be close..
PS - this method doesn't work on Cuda's...we got way more than our share for some reason?.
~ Pete