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: Cascadian Dark Ale #2: Empty
Secondary #1: Berry Rhubarb Wine #2: Empty #3: Empty #4: Empty
Kegged: Irish Red
Bottles: Surly Bender clone, Big 50 Barleywine, Framboise Lambic, Dark Belgian Strong, Kicked by a Moose Scotch Ale, Russian Imperial Stout, Sparkling Elderflower Wine, Barolo Wine, Peach Rhubarb Wne
On Deck: Spruced Winter Warmer