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.
Ex:
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.
A thermometer will only drive you nuts!