I don't know whether they are going under to stay warm, or going under to hide. Chickens often like to be under something (probably survival instincts--no hawk can get them if they are underneath.)
There are so many variables, I think it's often easiest to just try it, and watch/listen to see how they do (if you have enough time to do that.)
If outdoors is currently warmer than your basement, it's a good time to start, and then you can see how they do as the temperature goes down toward evening.
During the day, the air needs to be warm enough that they can run out to eat and drink.
At night, they will want to stay in one place and sleep anyway, so cooler air temperatures can be fine (assuming the chicks are sleeping under the heat plate, and assuming there is not a strong wind blowing on them to cool them off--I think your situation has both of those points taken care of.)
