6pm as it's improved my overnight readings. If I go to bed early, with a late dinner in my system, my readings can through the roof. I don't have a cutoff time for corrections. I just follow my usual policy of trying to correct as early as is safe to do so* (as the earlier I catch it, the smaller the correction required will be).
*For me, this is no earlier than 2 hours after eating and modifying the correction in relation to Bolus on Board.