I go by the one step warmer than me rule (DD sleeps in her cot now and DS is still wrapped to go to sleep), and still do. Because I sleep with the covers on and DD didn't, I put on a warmer layer, like if I'm just in a singlet and undies, I put light pants and a shirt on DD, if I'm in light jarmies, she's in warm ones with socks or a jumpsuit. It seemed to work for us.