I'd say 1.5 - 2 cups of water should be plenty. I added 4 oz of maltodextrine to a bitter once that finished too low. Worked really good and I feel like it saved the batch.
Primary #1: Big 50 Barleywine #2:Empty
Secondary #1: Peach Rhubarb Wine #2: Empty #3: Empty #4: Framboise Lambic
Kegged: Pumpkin Ale, Orange EPA, Piker Liker Porter
Bottles: Red Rye IPA, Dark Belgian Strong, Belgian Golden Strong, Kicked by a Moose Scotch Ale, Apfelwein, Russian Imperial Stout, Carmelite Tripel, Rhubarb Wine, Barolo Wine
On Deck: Umlaut my Kölsch V