I think it's a commonly accepted figure everywhere it's been. It was brought up in two separate press conferences by doctors in Oregon and Washington. That's why it's going to spread. 80% are mild and won't stay home. They'll spread it to others and 20% will end up getting really really sick.
I'd really be curious to see what the number is in Japan and South Korea, as their medical systems (especially Japan) used the US a model in standing up their healthcare systems (especially Japan as their system was wrecked after WWII).
I think the key is to minimize contact with others to the extent possible. Several of our staff worked flex shifts before this happened. Most of where they work has no people around. If people would not touch their nose, eyes, mouth, that also helps reduce risk.
But we were talking about this at work and several folks believe that we would shut down for a few weeks if even one person testing COVID-19 +. The main reason? Potential liability of remaining open and then someone getting sick. It actually would not be bad working there if the building is closed - there would be nobody around!