With a steel tank, full 3 mm wetsuit, 5 mm boot, a beanie 1 mm cap, I can dive with 6 lbs in fresh water.
With aluminum tank, same set up, I need 11 lbs in salt water.
With a 7mm/5mm full wetsuit, boots, and cap, I need 16 lbs in salt water.
With a 7/5 full suit, and a 7 mm corewarmer, I need 22 lbs fresh, and likely about 5 lbs more in salt.
With a drysuit, I need 22 lbs in freshwater with 200gm insulation.
With nothing at all, and a steel tank, I need no weight in fresh water.... But about 5 lbs with an aluminum tank.
You can not estimate weight needed unless you know the habitus of the diver. Obese divers have more fat, fat is buoyant, and you need to figure that in.
You also need to know the type of BC one is wearing .... Some are negative (steel or AL backplate), and some are positive (plastic moulded backplates).
The type of tank affects your buoyancy also. If you were neutrally buoyant, and dove with a neutral BC .... Steel tanks are negative when empty, while aluminum tanks are neutral to POSITIVE 6 lbs when empty.
You can not pass judgement until you know the type and manufacture of a tank. So called "neutrally" buoyant aluminum tanks are simply manufactured with an extra 4 to 6 lbs to compensate for the positive buoyancy characteristics of the AL tank. Unfortunately, this makes the tank heavier.
If you make yourself neutrally buoyant with your BC (unfortunately, we will ignore the reg), you can calculate how much weight you need to add or subtract by looking at the buoyancy characteristics of the following types of tanks:
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan