Don't get too hung up on details. The important thing is not how hot the heat lamp is. The important thing is the environment you are creating for the chicks.
What chicks need is a place they can go to so they can warm up if they get cold. Sometimes broodies take chicks out when there is snow on the ground. When the chicks get cold, they go to the broody to warm up.
I keep a 3' x 6' brooder in my coop with a good draft guard. One end has a heat lamp that is warm enough. I haven't put a thermometer under it in a long time to see what the temperature is where the chicks are. I know it is warm enough so that's not necessary. The rest of the brooder is well enough ventilated above the draft guard that it will cool off a bunch. I've had air temperatures in the 40's in the far part of that brooder but that other end is warm. Very young chicks play all over that brooder. They sleep pretty close to the heat in cold weather but when they are awake they are all over that brooder.
If I tried to keep an entire brooder one perfect temperature I'd go bonkers. Why make life hard when you can make it easy. Make a brooder that is warm one place and cooler other places and let them decide where they want to be.
If it is 100 degrees under the heat lamp, they won't stay under the heat lamp. They will stay somewhere the temperature is cooler as long as they brooder is big enough and well ventilated enough that they can get away from the heat.