How much weight to wear?

Test #1 (simplest): exhale at the surface

Test #1 (simplest): exhale at the surface

If you exhale your air on the surface, can you still float?

The "surface exhale test" ensures that if you black out on the surface and involuntarily exhale air, you don't sink.
Test #2 (more accurate): set your buoyancy to neutral at 10m

Test #2 (more accurate): set your buoyancy to neutral at 10m

99.9% of blackouts happen on, or within 10m of, the surface.

Therefore, if you set your buoyancy to neutral at 10m, you are almost certain to float to the surface in the event of a blackout.

Being positively buoyant will also make it easier to ascend the last 10 meters of a difficult dive.

To set your buoyancy: dive down to 10m, gently let go of the line, and check that you neither sink nor float. If you sink, return to the surface and remove some weight.
If you are diving at Lake Berryessa in a Fins and Foam wetsuit (or any other 7mm open-cell freediving wetsuit), bring 10 lbs of weight.

This is 4 lbs less than you would wear in the ocean with this wetsuit, because the lake water is fresh.

If you are a small slim person, or a dense muscular person, you may be able to use less than this.
— John Prins, Fins and Foam Instructor