Redsoxs has given you a great link. IMO, its just as easy to observe the behaviour of your chicks. If they huddle together and chirp, they are too cold, so lower heat source. If they avoid the heat source, even at night, then they are too warm, so increase the distance between brooder and heat source.
Bear in mind that the brooder is not supposed to be an even temperature. The idea is to mimic a mother hen and chicks, i.e. chicks will run under momma to warm up, and then scoot out and enjoy being chicks until they feel cool, when they will run under momma again. One area of the brooder should be warm, the rest should not be affected by the heat source.
Depending on the temperatures where you are, chicks should be able to manage without a heat source once they are feathered out - any time from 4-5 weeks onwards. You can "harden them off" after this time by putting them outside for short periods of time during the day - increase the period of time that they are out each day. Again, observe their behaviour - huddling together and chirping means that they are cold.