Room temp: 24-27 use 0.5 tog with short sleeve onesie
Room temp: 20-24 use either 0.5 tog with long sleeved onesie or 1.0 tog with short sleeve onesie
Room temp: 16-20 use 2.5 tog with long sleeved onesie

From the guide that came with my sleeveless sleeping bag.
I read this on the packet too, and then didn't buy them. Our bedrooms get down to 5* and less in Winter. Keeping bedrooms at continuous 16-20* is crazy. How did we all survive growing up?