if your backplate is actually 8lbs then you must have gotten it via one of the obscure manufacturers since they are usually 6lbs with the harness, but that doesn't really matter.
Buoyancy compensators do two things. Primary goal is to compensate for changes on buoyancy, shocking I know! The second is to potentially float a rig at the surface without you in it, very much a secondary function.
To float the rig at the surface, your lift requirements are as simple as summing up all of the stuff on the rig. Weight of the gas, buoyancy of the tanks with manifold/regs etc. Easiest way to do this is to take a luggage scale and just weigh it with the wing fully deflated when in the water with full tanks. You already have the wing, so you can just fully inflate the wing and make sure it stays at the surface.
Second bit is the sum of the change in buoyancy you expect to see and compensate for. Not sure what the 67's as I have never heard of them, but take the total gas they have and add that to the total amount of lead required to sink your suit. If in a 7mm farmer john, that's a lot. If in a 3mm, not so much. The wetsuit is typically a larger indicator of how big your wing needs to be than the actual tanks themselves as a pair of double 121's that are cave filled *hold 165cf of gas each*, only have a 27lb buoyancy swing from full to empty, but there are a lot of other things going on the necessitate a much larger wing when using those tanks
) I would have asked that question which there are plenty of threads about that already. So again thank you and have a nice day.
