I am guessing that your brooder may be too small. What are the dimensions?
The heat guidelines you're trying to adhere to neglect to point out that the temperature is measured just below the heat source and does not mean the entire brooder must be that temperature.
In fact, if the temperature guide were to be truly helpful, it would point out that there are, in fact, two temperature zones every brooder must have. The warm zone to warm the chicks, and a cool zone, a good twenty degrees or more cooler where chicks can self regulate by ridding their bodies of excess heat.
The 250 watt bulb may in fact cooking your chicks within the confines of a small space, effectively turning it into a Suzie Homemaker oven. Small brooders and those wicked heat lamps kill more chicks than any other risk factor during their first week of life.
Some of my friends here on BYC and I are on a quest to change minds about brooding chicks under dangerous heat lamps, and trying to promote the concept of the heating pad system (or the more pricey heat plates) instead. This system virtually eliminates the dangers of overheating, and it has other advantages as well, such as allowing the chicks to have natural day/night rhythms which reduces stress and gives their little bodies a rest during the night when they are doing most of their cell growth.
Meanwhile, consider brooding outside in your coop in a safe pen. ( See my article linked below on brooding outdoors.) Install your surviving chick in it with a heating pad cave. See the thread on heating pad brooding on this very page on this forum for a guide to how to set it up. The proximity to the other chickens will help the lone chick from being lonely until you can get two more chicks.
