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: Border Crossing Vienna #2: 2H IPA III
Secondary #1: Berry Rhubarb Wine #2: Black Currant Wine #3:Cider
Kegged: Orange Pale Ale IV, Hidden Lake Nut Brown
Bottles: Spruced Winter Warmer, 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, On Deck: Ice Shanty Bock