We don't have specific data for each of our suit sizes and thicknesses. If someone is curious, they might be able to place the suit in a mesh bag and add weights until it sinks. If you make sure to eliminate trapped air pockets, it should give a good estimate of the suit buoyancy. A brand new suit will have more buoyancy than one that is "broken in".
If the pool is warm enough, a diver who is curious, might also try to determine their weighting needs without a wetsuit or any gear at all. There is a good bit of variability in the buoyancy between individual divers.
There was also a question of how to use the freshwater weighting and translate that to saltwater. Since salt water weighs about 64 lbs per cuft and freshwater around 62.4, the difference is 1.6 / 62.4 = about 2.5%.
So you weigh (or estimate) ALL your gear (including your freshwater weights and yourself) and multiply that by about 2.5%.
So for a 200 lb diver with 10 lbs of lead and 60 lbs of gear, they would need 0.025 * (200 + 10 + 60) = about 7.
Around 7 lbs extra lead would be used in saltwater.
Edit: I just looked up another reference and (in the metric system) freshwater has a density of 1 and sea water about 1.025 (g/ml). So that also shows that the difference is around 0.025 or 2.5 %.