I use a system I call the "Magic Number".
It works for any given exposure suit once you have determined your neutral buoyancy, and allows you to change tanks or BCDs and maintain proper weighting.
Add up all the pieces:
Tank at 500 psi
First stage negative buoyancy
BCD buoyancy
Fin buoyancy
Trim weights
Integrated weight
Extras
Once you have a total, and you know your weight requirement, the Magic Number stays the same.
For example, with my 5mil wetsuit, boots and hood, my Magic Number is -12:
+3.4 Al Tank at 500 psi
-1 lb. first stage
0 BCD buoyancy
-1 old Jet Fin buoyancy
-4 Trim weights
-8 Integrated weight
-1 Light Canister Extra
Add 'em up? -12
With the same wetsuit configuration, if I switch tanks, the Magic Number stays the same:
-2.7 Steel Tank at 500 psi
-1 first stage
0 BCD buoyancy
-1 old Jet Fin buoyancy
0 Trim weights
-6 Integrated weight
-1 Light Canister Extra
Add 'em up? -11.7
If I switch to a steel backplate and wing, and say, switch my Jet Fins for floaty Nova fins:
-2.7 Steel Tank at 500 psi
-1 # first stage
-5 BP/W buoyancy
+1 new Nova Fin buoyancy
0 Trim weights
-4 Integrated weight
0 No Extras
Add 'em up? -11.7
It stays constant for the same exposure suit configuration. I have a short list of M.N.'s for each of my suits from rash guard to drysuit with various liners. But don't forget that at the beginning of a dive you'll also be carrying 5 or more pounds of air that you'll breathe off. So at the beginning of the dive you'll always be heavy, even if you're neutral at the end.
Why is the AL80 only +3.4 lb instead of +4.4 lb? Because the weights in the various tables are at 0 psi with valve. But you'll be carrying 500 psi (~1 lb) in your tank when you do your neutral buoyancy check.
