I would leave Mystic about a half hour before flood at the
Race. I’d use the flood
current all the way to Greenport but I wouldn’t go through Plum Gut, I’d go through The Sluiceway east of Plum Island.
Just be careful in The Sluiceway. The
current will be against you for a short distance and it will set you to the west. To pick a number your bow might be crabbed over as much as 40* but it’s better than Plum Gut.
Once through The Sluiceway you’ll have more flood current south of Plum island, all the way to Greenport.
That’s how I’d do it. For whatever that’s worth.