You need a little tutorial in why we provide heat for baby chicks, then you will know when to turn the lamp on, when to turn it off, or when to use a smaller wattage bulb.
Baby chicks are only wearing fuzz until around four weeks old. Then they have grown in most of their feathers so they are able to insulate against heat loss from their bodies better. Until then, the fuzz doesn't allow them to retain body heat when the ambient temperature is lower than their body temp. So they need a heat source under which to warm themselves and replace that lost body heat.
When the ambient temp is quite warm, close to their body temperature, the chicks aren't going to lose body heat faster than they can replace it by eating food and converting it into calories. Therefore they don't need a heat source under which to warm up when the day is very warm.
You've noticed the chicks are avoiding the heat zone on warm days. That's your cue to turn off the lamp or reduce the wattage of the bulb. Continue to observe the chicks and increase the heat again as the ambient temp cools down and you see the chicks moving back under the heat source and huddling for warmth.
