Before deciding about the amount of fruit per gallon, I would suggest looking over several recipes for wines from that fruit, including Keller's. Not ALL fruits are best with an all fruit, no water approach. Many are but certainly not all. Elderberry is one that most folks I read about claim that elderberry can be excellent with ad little as 3 lbs/gallon. Blueberry on the other hand, one of my favorites since I grow them, can be overly acidic and difficult to get started if you do an ALL FRUIT recipe. Investigate and then as Vinobeau suggests do a couple of different quantities of fruit at the same time. (And mark the carboys well...) As always - "Your results may vary."