First off a saltwater pool that even approaches the salinity / specific gravity of ocean water is very rare so for this exercise consider it to be fresh.
You have established that ALL gear is going to be the same so that's that.
It comes down to the stated difference in specific gravity so unless your someplace really odd like the Dead Sea the 3% of total dry diver weight is what you are looking at. This is your total weight before you jump into the water, you suit, equipment, full cylinder, weights, everything.
Keep in mind that your excitement, surge, current and a host of other factors including discrepancies in your prior weight check can all effect the end result. Add the 3% and try to bring some as small weights that are clipped on or in accesible pockets. Near the end of the dive in 10-15 feet of water with your cylinder near 500 PSI and your BC empty of air you can hand off small amounts of weight. You will know when you get too light, take that last piece back and you have a really dialed in number.
Pete