How to know what to dress baby in?

[Frenchyfancy]

There are so many different opinions on this and the charts online vary so much!
How would you dress your 4 month old in 20 degree bedroom?

Short sleeve vest, babygrow, 2.5 tog sleep sack. You're just on the upper limits of room temp for 2.5 tog sleep sack, so if they feel warm to you then lesser tog. Does the room temp stay consistent overnight? It's usually 20 degrees when I put my DS to bed, but drops to 17-18 over night so definitely needs the above layers. If its 21 degrees or more, I do the same but with a 1 tog sleep sack.