I agree that trial and error is sufficient. I included the calculations to provide a scientific basis for what most responders were saying by experience. I wanted to show that buoyancy changes are affected not just by a change in dry weight, as most responders here seemed to be suggesting, but also by a change in volume (only one poster mentioned volume by name). Your example brings up an important point about buoyancy that is not mentioned very often: body volume. The fact that she weighs a lot less than you implies she should use less lead weight (that's the W in the equation). If she's bigger around (no offense meant) than she makes up for the loss in buoyancy by water weight displaced (the VD in the equation) and has to use more lead weight to compensate. Hence, the small difference in lead weight used between you and her. Taller, thinner people will be tend to be less buoyant than shorter, bigger people.
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