There are two reasons I’m familiar with to put up protection like that, to keep snow out of the run so they have access during the winter and to block cold winds so they will use the run. Mine enjoy a warm breeze but really hate a cold wind blowing on them when they are in the run or out foraging. Mine will forage in below zero Fahrenheit weather as long as it is calm but if the wind is blowing they seek shelter from the wind. I haven’t paid a lot of attention to exactly what temperature they start avoiding a wind. Part of that is how strong the wind is but I’d say they start avoiding a wind probably around freezing.
But you have a different situation. Yours sleep in your run. I’ve seen chickens sleep in trees at zero degrees F, but those were in a protected little valley in a thicket and they could move around the tree trunk to get extra protection from the wind if they needed to. Yours are going to be out on the open with no protection except that plastic. Wind chill depends not only on the temperature but on the wind speed. Humidity is also a factor.
We normally worry about frostbite, not them freezing to death. Frostbite cannot possibly occur until the air temperature drops below freezing, so there is one limit for you. But those chickens sleeping in trees in zero weather did not get frostbite. There are other factors than just temperature involved, including humidity. Those chickens in the trees had great ventilation and did not get frostbite.
I don’t have a hard and fast number for you. What’s wrong with putting it up now? What will it hurt?