A 3.5 TOG sack is the warmest in the makers' charts, for a room below 16°C. The Baby Layers app never suggests it: in a cool room it keeps the 2.5 TOG sack and adds a sleepsuit under it.
Under 18°C: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG).
The sack makers' charts give 3.5 TOG for a room of 16°C. The safe-sleep charities point to those charts, and your sack's own label wins over any table.
Below 18°C, the app keeps the 2.5 TOG sack and adds a sleepsuit under it, over a long-sleeve bodysuit. The Lullaby Trust says a cooler room can take an extra layer of clothing or a warmer sleeping bag, and the app takes the layer.
Nothing, not the baby's age and not a parent's setting, ever makes the app's sack warmer than the chart. A baby is safer slightly cool than too warm.
If you use a 3.5 TOG sack, follow its own label for the room and what goes under it, and check the neck once your baby is settled.
Never a hat for sleep — a baby cools through the head.
A sleep sack, never a loose blanket.
Safer slightly cool than too warm: nothing makes the sack warmer than the chart.
Check the neck — warm but not sweaty means just right.