Different wattages produce different amounts of heat. For example, with regular layer breeds, I start with a 100 watt in the lamp and use that for the first week, then decrease to a 60 watt for a week or two, then down to a 40 watt, and finally a 20 watt. This allows me to slowly decrease the temperature in the brooder without having to raise or lower the light. I watch the chicks to see how they are responding. If I change down the wattage, and they all huddle under the middle of the lamp and cheep loudly, I know they weren't ready yet, and I put the higher wattage one back in. If they are in the cooler part of the brooder and panting with mouth open, then I know I need to decrease the wattage because they are too hot.
I know with the cornish X I've raised, I see a lot more panting early on, so the wattage might go down pretty fast. I might only use the 100 watt for 2 or 3 days, then go down to the 60 for a week, then the 40 for a week, and finally the 20 by the time their 4 weeks old. Mostly, I just watch the birds and see whether their too hot or cold according to their behavior, and adjust wattage accordingly.
Please keep in mind, this is for brooding in my house, not outside in the garage or in the basement, which would affect the heat requirements.