This is my 8th or 9th year raising chicks and while I will not admit to being perfect at it, I will admit that I haven't lost a single chick (well, I did lose one 6 or 7 years ago that I rescued from our feed store that had a crusted over rear end - I tried to save it but it didn't make it).
I have never used a thermometer, and I have probably never had my chicks at just the right temp. We do live in a warm climate (Louisiana), but I have had chicks as early as March 1 and have had no trouble. I just have a heat lamp they can get under, and plenty of room for them to get away. They must be okay because they are rarely huddled under the lamp. They are almost always scattered around the brooder picking away at stuff.