Try putting down a thermometer on the floor of the brooder half way between the center of the light reflector and the wall of the brooder. The first week you have chicks, that temp should be between 90 & 95 degrees. If a 100 watt lamp will give you that result, use it. But unless your brooder is really small, Iʻm guessing youʻll want the bigger lamp. They cost the same to purchase, you need the same porcelain fixture for each, only the cost of the electricity goes up with the wattage. Each week thereafter, raise the lamp until the temp goes down by 5 degrees. Again, this is easier with a 250 watt lamp rather than a 100. If the chicks are clustered under the light, its too cod for them. If they are spread evenly around the brooder doing their chick thing, its probably just right. If they are up against the walls, its too hot and the light must be raised.