I make my batch size same as my wort size, and then in the "water needed" section set for that same volume in the field "after chilling" to fermenter(s). With a little trial and error, the rest for me is setting the efficiency setting in the "edit Ingredients" section so that I get the volume and SG I'm shooting for. At least this is the basic approach for me, and works most accurately when I have few losses from wort left behind in the mashtun/kettle/other also as figured in the "water needed" section. I often refer to past batches to recall what efficiency to expect based on the predicted OG.
edit: What that wort size volume is depends on how much beer I am making, usually 11 or 12 gallons.