Warren_L:ONe thing to keep in mind is that you should not be basing this calculation on counterlung volume, but rather loop volume. When diving rebreathers in general, you want do dive with what we call minimum loop volume, which is essentially your lung volume (which is part of the loop), empty (or near empty during inhilation) counterlungs, and non-compressible air spaces (hoses, scubber canister, etc). So factoring full counterlung volume is not that really relevant.
Think of say an empty lift bag you might carry with you. At depth, if you fill it with water, will your buoyancy change? Same sort of thing.
what is a reasonable estimate for someone is about 4l of cl volume and 4l of dead space or about an 18lb swing.. newer RB divers tend to have more volume in their lungs than an experienced diver, From observation and experience howmuch weight a RB diver drops from his rig, double that of an experienced diver is a reasonable estimate (8 l instead of 4, so 12lin total or about 27 lbs)