Bail-out is profile dependent as well as team based for most longer rebreather dives. If you figure a 60 minute bottom time - say 30 minutes at 120' going out and 30 Minutes at 90' return, a pretty common wall dive... you would only have around 10-15 minutes of deco on the breather but if you have to bail-out you could be looking at 45-60 minutes deco on air. (V-Planner)
At Zero Gravity we were planning 1.5 to 2 hour run times on the rebreather, so bail-out requirements really were team based. I dived a lot with Gordon and Kim, I think they were slinging 30's or 40's and diving as a pair and often with others nearby should a need arise. I don't think anyone had to bail-out the entire event. The majority of divers at ZG were diving much deeper 200-300' and would sling two 30 or 40's bottom mix/02 for redundancy and bail-out. A few folks would sling 80's on deeper dives. The deep divers would definately have needed a team approach to bail-out.
So depending on Conservative settings, RMV, stress levels, etc... a 30-40/ft MIGHT be enough, but certainly two or three divers in a team would be able to support one bail-out scenario with a pair of 30's or 40's. A 19/ft might be fine if you really limit your profile to no-deco... but as soon as you have wracked up an obligation of more than a couple of minutes on the rebreather the safe bet is go with more bail-out rather than less.
I always like to plan to have enough gas on me that I can "get out" without anyone else's gas but it is not always practical. It's always a bit of a surprise when you run dive planning software and do bail-out scenarios just how much gas you should be carrying for bail-out... because we never dive deeper or longer than we planned do we>;-)
Doug