Logically speaking they have to allow last minute form submissions because someone living in Blaine or Point Roberts could literally be less than 1 minute away from the Canadian border. So you should be fine doing it at the very last second though keep in mind that there’s barely any cars waiting in line to cross into Canada meaning that you won’t have enough time to do it at the border and there’s nowhere to pull over once you’ve entered the final border crossing area. If you’re driving with someone in the car, have them do it roughly 10 minutes before the crossing. If you’re alone, pull over at the last gas station and get it done.
Though personally I’d just do it the night before, giving them an approximate time of arrival. The limit is only there to ensure you’re not over the 72 hour limit, there’s no actual rule saying you must arrive on that exact moment.