Filling the tank water to ~8-9oz gives a water:coffee ratio of ~16:1 for a recommended, "golden cup" coffee ratio.   However, if you prefer stronger coffee, put less water in the reservoir, down to 4oz.  You can also experiment with adjusting the coffee amount and grinds size.  Just make changes incrementally as if too  fine grinds or significantly greater amounts of coffee are used, there is a possible risk of overflow during the brew.