This isn't an exact science and doesn't scale linearly. Also, it's a made up concept. Chicks raised naturally by a broody hen don't start life with their whole environment warmed up to 95 degrees. They usually hatch in the spring, when the weather is still quite cool, and go about their business outside as soon as they hatch, only going under their mom to warm up occasionally. You won't get 95 degrees ambient temperature unless you live somewhere in the tropics or happen to time all your broody hatches in the middle of summer (and even then, not everybody gets 95 degree summers, and yet chickens live and raise young all over the globe). So the number 95 (or 90 or 80 or whatever) is made up. The whole artificial brooding in a box with a lamp is a human concept, trying to mimic nature with the tools that were available at the time. Now there are better methods - like heating pad / brooder plate type setups, where the chicks go under to warm up and then come right back out, at normal ambient temperature (your room, or your coop, etc.) This is much closer to what they'd have with a hen. As hen-raised chicks mature, they need less and less heat, so they spend less and less time under her. The gradual lowering of your brooder temperature is meant to mimic that. But it's hard to figure out when to do it, by how much, etc. Those numbers are an approximation to use just as a guideline. The chicks know best. That's why the heating pad / brooder plate method is better. It lets the chicks decide when they need warmth.
In any case, supplemental heat is only necessary until their proper feathers grow in. Real feathers can keep them warm better than baby fluff. So by the time they have all their feathers, they are ready for the world and the weather.
