I'm not sure I understand the reason for 3rds, halves, etc., when solo on CCR. I tend to carry my BO/stage bottle(s) and drop them where I will use 80% - 90% of that/those bottles to get out. I then work around the last bottle dropped, the distance from the last bottle being based on the BO I have on me. I'm a little different than most, in that I've got 2 steel 45s pumped on my canister (see the yellow tanks in my avatar) & 1 or 2 alum 40s sidemounted. There's also an AL40 at the cave sign with 100%. All bottles can be plugged into the RB.
I'm comfortable that I can get out safely with the BO I carry in. Most people think I'm nuts to take so much gas with me, but it's my comfort level that is the only thing that counts.
Rebreathers are something that we have to go to in order to dive the way we want to dive. They are not something we go to for any other reason.
When I backmounted I usually did 1/2+200 on a stage without using back gas. In sidemount I just dive 1/3's with a 200-300 psi cushion unless it's gnarly wiggle type passage.
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