At 3 days old they need to be at 95 degrees. Once they are about 7 days old reduce it to 90, 2 weeks old they should be around 85.
Mine will be 4 weeks on Wednesday and techinically should be at 80 degrees since they are still 3 weeks but it has gotten to 72 and they still appeared content.
So play it by ear and move the lamp up and down as needed.
Since I am in Maine, I have to use 2 lamps. It's 33 degrees outside. Inside the coop is around 54 and inside the brooder it's 77.
We turned 1 lamp off the other night, hubby went in in the morning and it was only 67 in the brooder. They were obviously cold since they were on top of each other. Turned on the 2nd lamp and it no time they were walking around and eating.