Granted I'm very new to this, but I stirred 2 cups sugar in to 2 cups of inexpensive Pinot Grigio until it was clear, and used that to sweeten some sour cherry and concord grape and a blend of these. For a batch of DB, I added the sugar for all 5 gallons (3.75 cups) into a bottle of another white with a similar taste profile to the fruit I used in the DB (ie. a wine with tropical notes for a pineapple/banana/strawberry/mango batch). Both times the wine stayed super clear. We found that adding the sugar brought the fruit flavour forward and made it much more drinkable (even before aging!). Now just trying to figure out timing from back sweetening to bottling- should it sit in carboy with the sugar for a while, or can it do all its flavour melding in the bottle?