For a gentle one hour bimble I use an Al 80, for a slightly longer dive I switch to an Al 100. For deeper dives (15-30m) I switch to a steel 15litre and side sling an AL40. Anything other than a "bounce" below 30m is done on twin (usually manifolded) 12litre steels usually with a 7litre side slung(50% O2) (Deep to 40m I'm on trimix)