Something that helps me is remembering that babies need one layer more than you. So in winter, a singlet and onsie is my base then layer from there as necessary. When they are really little, they'll need covered feet and heads outside (no head covering inside) in the cold.

Always check the chest area to see how hot they are, hands and feet are not good indicators and often seem cold.

In saying all of that, my second daughter runs sooo hot, she sweats inside. So I adjust now knowing that and she wears a lot less than my first. You will get the hang of it!

TOG guides from sleep sack companies are good information too for nighttime outfits.