I've always used a temperature chart to help me know how to dress my baby, but they can differ quite a bit.
We have a Tommee Tippee GroEgg and I tend to use the temperature sleep guide that goes along with that. Tonight in our room its 21.3°C, so I've dressed my baby in a 1 tog sleeping bag and a short sleeve vest. I keep noticing though, that other guides recommend different layers underneath. For eg, Babycentre recommend that for 18 - 21°C, you layer a vest and a sleepsuit under a 1 tog sleeping bag.
I know they're just guides and every baby is different, but I'd love to hear which you'd gravitate towards more for your baby?