From mostly reading about when other people start having issues and when they have no issues. And what people have seen chickens do when they have a choice (and whether their choices worked well.) And partly from one winter of experience with my chickens...
Assuming they have good ventilation... at the very least, 1 square foot (1000 sq cm) per bird all the time, especially when it is cold or damp.
45F (7C) to start being a little concerned but not panicked. So, check that the tarp is deflecting most of the wind and rain away from them if it is a strong wind or moderate or heavier rain. I would want them to have such a place at any temperature but it starts needing to be available at about this temp.
35F (2C) to reach urgency in checking they have a dry place away from a strong wind to go to if they want to.
I'm not sure where the panic threshold is; their outer feathers are surprisingly weatherproof. If they are soaked to the skin, it is a higher temp than if they are dry or only somewhat damp. If there is enough wind to part their feathers and they can't get out of that wind, then it is a higher temp. Days on end vs a few hours of less than ideal exposure, than it is a higher temp. If they have some sort of health issue going on, it is a higher number. If more than one of these, it is a higher number than if only one.