Two basic comments:
- 250-watt heat lamps are far too hot for small batches of chicks, even in the first week -- unless it's below freezing in the room. Maybe even then. At 25 °F, a 250-watt heat lamp is still good for 50 chicks, so a 125-watt heat lamp is good for 25 chicks in a freezing room!
- Thermometers measure air temperature, but are poor at measuring the effect of radiant heat from heat lamps. And the radiant heat is what matters most. The recommendations I've read with radiant heat range from, "Don't use a thermometer at all: just watch the chicks' behavior" to "Stick a piece of black electrical tape to the bulb of the thermometer, so it responds to the heat lamp and not just the air temperature."
I saw a photo of demonstration where a dozen chicks were brooded with overhead heat lamps -- in a walk-in freezer kept at a constant twenty degrees below zero. They used (I forget) either three or four heat lamps. It was so cold that ice formed on the shadowy sides of the waterers, and there was no brooder box or draft guard to hold in any heat. But the chicks were perfectly happy and comfortable inside the beam of the heat lamps. (I really must find that issue ... the picture was great.)