Does anybody know the water calculation for high gravity brewing? I need to make 15 gallons into my keggle, obviously this wont work so I need to up the gravity then dilute in the fermenter, I have never done this and haven't a clue how much water to add back before pitching the yeast, aside from adding a bit at a time and taking gravity readings along the way, which seems 1) unreliable across 3 carboys and 2) a pita. Any advise is appreciated!