12C+full summer gear
8-12C - Bib Shorts&thinnish long sleeved top, possibly thin gloves
6-8C, dry: -Bib shorts and thicker long sleeved top, thin gloves. Possibly leg warmers with shorts.
6-8C, damp: Bib tights (or possible shorts with leg warmers) & long sleeved top, thick gloves
4-6C: Bib tights, long sleeved top and possibly wind jacket (though might omit wind jacket if humidity is low). Might go for double socks, or neoprene overshoes depending on which way I think the temperature will go. Thick gloves.
4C and below: Full winter gear: Bib tights, long sleeved top, fleece lined jacket, and if it's very cold wind jacket as well (so 3 layers on top), Neoprene overshoes. Thick gloves.
These are commuting rules for me because it always gets warmer on the way into work (I go through london and it's always warmer there). If I'm heading out in to the countryside, I might go with an extra layer from 4-10C as the temperature tends to go down as I head further out, or I'd at least shove a wind jacket in a small rucksack.