When considering what level of heat your chicks require, it's helpful to wrap your mind around the "why" of it. First get rid of the notion that chicks are like cakes baking in an oven. The analogy is more like a bunch of little campers needing a camp fire to warm up at.
The temperature chart for brooding chicks is only a starting point to let you know approximately what level of heat you need to begin with. Then pitch the chart in the trash bin. From then on, you will judge what level of heat your chicks need by observing their behavior. When they act chilled, increase their heat. When they act like they're too warm, reduce the heat.
Chicks generate their own body heat from consuming calories, as we do. And like us, they lose body heat commensurate with how cool their environment is and how much feather covering they have at each stage of development. The less feathers, the more the heat loss. The cooler the ambient temp, the more the heat loss. So they need a heat source to replace the lost body heat.
Chicks usually have enough feather growth by age three weeks to keep from losing any body heat at temperatures of around 70F(25C) So they need no heat source. At night, if the temperature drops below that, they will require some heat. Keep in mind that during the day chicks also are consuming calories which helps prevent heat loss, so their heat needs increase at night because of that.
Most folks heat their chicks far too long, and this prevents acclimatizing to cooler temps and inhibits quality feather growth. The best heat guidelines are the less heat, the better.
Between ages four weeks and six weeks, most baby chicks are almost, if not completely feathered out, the equivalent of wearing puffy down jackets, and they no longer require a heat source.
