Tobin at Deep Sea Supply
https://www.deepseasupply.com/ is the pro at answering this question, but basically:
You need to determine how much buoyancy your exposure suits provides (this is the buoyancy that can potentially be lost), the buoyancy of your rig with full tank(s), and ideally your personal buoyancy (which is positive for most folks and can't be lost at depth).
One way to determine the buoyancy of your exposure suit is to get into neck deep water with your exposure suit and a mask on, have a friend hand you weights (hold them in a mesh bag), lift your feet, and see how much weight is needed to achieve neutral buoyancy at the surface (floating at eye level with a normal held breath). Or, you can stick the exposure suit in a mesh bag, add weights to the bag until it just begins to sink. Once you know the buoyancy of your suit, you know how much lift it provides, and how much lift you potentially can
lose at depth... this lost lift needs to be compensated for by the BC. At the beginning of the dive, if properly weighted, you will also be starting at least several pounds negative (the weight of the air in your tank).
Your BC needs to provide enough lift to offset the lost buoyancy at depth, taking into account the buoyancy of your rig with a full tank and the ballast weight you are carrying. It also needs to provide enough lift to float your rig on the surface if you take it off (thus removing the lift your exposure suit provides).
In your example, if you assume your wetsuit lost "all" of its lift at 120ft, you are lean and have no "personal buoyancy".... using an AL80 that is about -2 lbs full, plus another -3 lbs for regs, -5 lbs if using a steel back plate, plus -21 ballast weights sounds like you'd need about 30-35 lbs of lift minimum at depth as a wild ballpark guess.
Others may be able to give a much better estimate, but you will need to know what tank (I just used an Al80), BC, and what the exposure suit buoyancy is....
Best wishes.
