That is an extremely conservative rule of thumb, I've had chicks thrive on less. Chicks can be like people. If you have a meeting room at the "perfect" temperature some people will be a bit cool, some a bit warm, and some just right. But if you follow that rule of thumb they will all do fine...
I use a heat lamp, not brooder plates. Some heat plates say to not use them in less than 50 degrees Fahrenheit (10 C) temperatures but some don't. I'd check your manufacturers recommendations or maybe send them an email. I'd expect it to be a lot less of a problem if you have decent draft...