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: Empty #2: Empty
Secondary #1: Berry Rhubarb Wine #2: Black Currant Wine #3: Ice Shanty Bock
Kegged: Border Crossing Vienna
Bottles: 2H IPA III, Carmel Apple Cider, Spruced Winter Warmer, Big 50 Barleywine, Framboise Lambic, Dark Belgian Strong, Russian Imperial Stout, Sparkling Elderflower Wine, Barolo Wine, On Deck: ??