We work by the nursery temperature. Here, it's often 26 when she goes down for the night, so I dress her in a singlet suit and just wrap her in muslin. When she wakes for a night feed, I pop her into a sleeping bag (1.0tog) then for the rest of the night.

I think winter will be a lot harder though as the temperature drop is a lot more overnight so it would be difficult to decide how many layers to put on them.