You are pretty much at the mercy of your refrigerator if that is what you are using to store your dough balls during cold fermentation. So, it is difficult to talk about "exact" temperatures. Commercial coolers run about 35-40 degrees F. If the cooler is used primarily to store dough balls and especially if the dough balls are made in the evening such that they remain pretty much undisturbed during the night, without workers going in and out of the cooler, the cooler temperature will be fairly stable. A home refrigerator on the other hand runs around 40-45 degrees F and is subject to frequent opening and closing of the door to add or remove contents. That can cause the temperature to vary more than with most commercial coolers, with a bias toward the upper end of the refrigerator's operating range. In your case, with a desired 24-28 hour cold fermentation, I don't think you have anything to worry about. Where you are in Canada, especially this time of year, a 25-minute autolyse of 70% of the flour at room temperature is not going to materially affect the finished dough temperature. I don't think it would be a problem even in the summer if you are using 24-28 hours as a target and you aren't using high levels of yeast.