I do not really understand what those "cf" mean. I use a 3 liters pony at 200 bar (600 liters total).So what’s the optimal size for a back
mounted stage used to extend the dive at 100ft? Is a 9cf enough or do I need a 13cf?
Typically I attach it in the middle of my twin tanks (2x 9 liters at 200 bar), or beside my single tank (15 liters at 232 bars). In both cases the main tank has two independent valves, two fully independent regs and reserve.
This is a good redundancy, in my opinion, for demanding dives (when crossing the border to tech, which in reality I did in the past only a very small number of times and I am not doing anymore in the last 25 years).
But here we are in "basic scuba" forum, so I understand that my pony setup is "overkilling" here...