The polar that is loaded in the computer should include the flying weight of the glider without ballast (empty weight, plus pilot, plus parachute, sandwich, etc). You just need to then tell it how much water you are adding. I think the easiest setting may be "weight". Then you just enter how many lbs of water you are loading and you should be good to go.

Be sure to download the latest firmware if it wasn't shipped with it. I had to do 2 updates to get mine working in the last 6 months.