By ALL means put a thermometer under that lamp before you put chickens! If the chicks are a week old raise the lamp until the temperature reads 90 degrees. Thats where you should start. If the chicks huddle together underneath the bulb, then they are too cold and you should lower it fractionally. If they scatter away from the bulb, then it is too hot and you should raise it. For every week of age lower the temp under the bulb 5 degrees by raising the lamp. After about four weeks you should be able to do away with the bulb entirely, providing that the night time temperatures do not fall below 60 degrees. If they do, then you should have the lamp on at night when temps fall below that level. After six to eight weeks they shouldn't need any lamp at all.
I feel that 250 watts is too much inside a house unless you have a huge brooder with more than say 20 chicks. With 10 chicks I would use 100 watts. With my five chicks I used 60 watts and it was plenty.