In order to find out how much water you actually have in your tank after adding live rock you need to determine the average specific weight of the live rock in your tank. I don't think vendors usually have this info so determining the amount of water displaced by the rock is difficult. Experimentally you can fill a 5 gallon bucket, or any size bucket for that matter, full to the top with water. Add live rock to the bucket so the water overflows over the top. Take out the live rock and measure how much water is left. The difference is the amount of water the rock displaces. Further more if you know you added 30 lbs of rock and 2 gallons overflowed....well there's your specific weight. 30/2 = 15 lbs/gal. If all your rock is similar you can take this figure to determine how much water displacement you have with any amount of rock.
I think the easiest way to determine the weight of your aquarium is to just be conservative. I'd add the weight of the water plus the weight of the sand, rock, tank, and stand. Your final figure will be a little more than what it actually weighs but that's always a good factor of safety. Plus, unless you're accurately weighing the rock each time you add it and keeping records, you're pretty much guesstimating the amount of rock you have anyways.
At 20 degree's celsius, the specific weight of pure water is 8.33 lbs/gal. Multiply that figure by how many gallons you have and then multiply that by the specific gravity of your water.
Example:
55 gallons of water with a specific gravity of 1.024 will weigh:
55 x 8.33 x 1.024 = 469 lbs
Hope this helps!
