lead weight question

[freedivernd]

o.k., you all know I don't scuba much but I'm thinking I might go scuba diving ... maybe once, this winter. salt water.. maybe...
anyway.. can anyone tell me how much lead is needed to offset a steel or aluminum tank and bc.. I don't know.. maybe the standard rental size at most dive shops around the carribean what ever that may be.  Tell me salt water or fresh.  I've a pretty good idea what I would use in fresh water.. but can't remember what I used the last time in the salt.. no clue.  I do know it wasn't quite enough as I had to swim a bit at the deco stop to stay down.

I know, I know.. that's a pretty general question but I'm only looking for a somewhat general answer.  just curious about offsetting the tank and bc.  any ideas (don't say stick to freediving or any other smart remarks please.. I know you're all nice people!)

Fred