I use a red brooder light and a white brooder light (250) and I also use a ceramic heater made for reptiles. I use them at different times. When I need light and when I don't need any light. I use them at different stages of growth.
I have the white light and the red light on at the same time. When the chicks are a week or so old I turn off the white light. If the temp goes down to much I use the heater to keep it warm. My setup is that after a few weeks (5 or 6) I leave my door open with only the ceremic heater on. As the sun sets it slowly gets dark. They soon feel ok in the dark at night. Everything is gradual.
It depends on where your brooder is. If it is somewhere not heated you will need a 250 heat lamp bulb. If it is in the house where it is warmer, you won't need on that high of wattage. Just put a thermometer on the floor of your brooder to make sure you get the appropriate temps.
I would think that the correct wattage would ONLY directly depend on the SIZE of the brooder in question.
What difference does it make whether there are 5 chicks or 500?
What works for one person may not be exactly what everyone else needs.
I have 2 brooders going, 1 is 4x8x2 and needs a 250w bulb to work properly, there are 35 chicks in it.
The other is a 40 gallon fishtank with a 100w bulb, with 10 chicks.
The fishtank is in the unheated garage, but if I were to put the 250w bulb over them theyd cook, ans also if I were to use the 100w in the big brooder, theyd all freeze.
The best indicator of correct temps needs only to be a visual confirmation that the chicks arent piling up directly under the light, or trying to escape from it.
You want a nice even spread of chicks under WHICHEVER LIGHT SUITS YOU BEST.