A wetsuit diver could lie on the bottom with an empty tank and no air in the BC, yet still need to struggle to keep from rising to the surface at 15 feet as the suit regains its buoyancy. In my opinion it is best to do a weight check at 15 feet with a near empty tank in the same type of water you want to do the dive. However, I was taught to check using fin pivots and floating on the surface at eye level neither which really gives a good idea of weight needed during the safety stop.
Fair point, I was thinking of a particular case in a cenote. The bottom was probably only 3m by the steps.
My point is that no amount of adding up or thinking is a substitute for getting in and dropping weight till you can only just stay down.