I think you find that weighting is necessary only for the shallowest part if the dive, descending the first few feet, and then being neutral for a safety stop. Depth of the dive is not a factor, unless you are doing an entire dive at 20 feet or shallower. In such a case, I will add about 2 pounds. Remember to do your weight check with a mostly depleted tank, or else add 3 pound to what is neutral buoyancy with a full tank. All things considered, I would rather be a pound overweighted than a pound underweighted.
DivemasterDennis
DivemasterDennis