Edit: I missed the part about stressed SAC rate. What is a good value to use? Double a resting SAC? Higher?
I's different for every person, but likely you'll need more than double your
resting SCR. Most people are not resting when they're actually diving, and most people use double their normal diving rate for their stressed SCR. Even low-stress tasks, such as leading a dive, or performing well-practiced drills, make a noticeable impact on my breathing rate.
Add to that the fact that when you're really stressed, it's amazing how much over double working SCR you can actually go, and how incredibly involuntary that is and difficult it is to get under control. Another aspect is that people tend to compete with themselves to lower their SCR and, like trap times at a drag strip, oftentimes start to think of their lowest resting SCR as their typical SCR. IMHO, in terms of your
emergency gas supply it makes more sense to skew conservative than liberal. At the very least, I'd go with double with your actual working SCR. I can hit 0.4 on calm dives, but my highest working SCR is closer to 0.6. I use 1.3cu/min for my emergency SCR based on conservatism and actual experience. A typical number you'll see here is 1.0, but I imagine many of the people using 1.0 have more years of diving and more comfort in the water than I do.
This also dovetails into the whole "theory vs. practice" debate. A lot of times we use seemingly reasonable assumptions when working the numbers. Sometimes it works out; somethings we may be caught by surprise when the reality of the situation differs from what we've calculated--hopefully we haven't put ourselves at the very limit of what we can handle when that occurs. A lot of people are smart and/or diligent and will re-calculate or confirm their numbers when they have new or more accurate information, but I only mention this for the benefit of those who may be skimming the thread and only take away "19cf makes me good to 130ft" without thinking through what it means or whether it's realistic for them.