Fisheater is mostly right.. The density of salt water varies depending on where you are in the oceon. His formula works if the relative density is 1.025. However, you can check this on the internet, and adjust the formula accordingly. The reason why the formula works so well is that if you are neutrally buoyant, you weight exactly as much as the weight of water you displace. To adjust for seawater, you are simply adding weight (assuming of course that the lead adds negligable volume compared to its weight) so that you weigh as much as seawater you displace instead of freshwater. The formula actually accounts for all variables. Just step on a scale with all your equipment when it is completely dry, and calculate away. You should do this with an empty tank, as you should be neutral with empty tank, and not a full one.
If, howerver, you are changing your equipment (like wearing a wetsuit instead of a drysuit), this formula won't work. In that case, I suggest you check your buoyancy with the equipment you will be using in a pool, and the apply the formula.