Let the chickens tell you if it is hot enough. If they retreat under there when they get cold and they settle down and fall asleep, it should be warm enough.
I would much rather gamble with the heat of the heat plate when the ambient temoerature of the room is adequate for them than risk...
If you are worried about keeping them warm without a heat lamp, consider getting a heat plate. They can get under it when cold and out from under it when too hot. Like a mother hen. Then you will have happy chicks.
There have been people who have killed their chicks from them getting too hot...