Okay, the picture linked is the formula I used to determine how many gallons of water my tank held. It requires a change in the salinity to work. You know the current salinity of your tank, you know the amount of water you removed, you know the salinity of the water you are putting back in, and you know the final salinity when everything is said and done. Using those knowns you solve for the unknown, the total water volume in your tank. This formula would account for any additional equipment you have hooked up to the system. The output is the amount of water actually in the system after all displacement is accounted for.