You can assume that the brown sugar will ferment completely. Take your original gravity reading, and your initial volume at that reading. Go to the Fermcalc
(I use it for port wine syrup feeding and also for fortification calculations) , and enter in these values in the Sugar
As an example, I just plugged in 1.065 as the original gravity. The resulting adjusted starting gravity after adding 1.5lbs brown sugar+1qt water after fermentation has already begun, assuming 5.25G original volume, is 1.073 @ 5.61G new volume.