Experimentation and ideally a selection of options to choose from... I thought I has a system nailed down while gravel riding and now MTB'ing I find I'm overdressed for the same conditions.
For me, the speeds are lower and exertion levels are a bit higher... so having windproof materials is less useful (generally hotter/sweat'er) and dressing less.
I have a mix of:
- base-layer shirts (sleeveless, short sleeve and long sleeve) in poly and a short sleeve in wool
- socks (thin poly, thin wool, medium wool)
- bibs (undershorts, shorts, thermal tights and windproof thermal tights)
- MTB over-shorts
- long and short sleeve poly jerseys
- long sleeve thermal jerseys
- cycling jacket (thermal and windproof front/arms)
- headband and skull cap
- gloves (thinner, medium, windproof, etc..)
- platform and clipless shoes
- toe covers for clipless
So far the coldest I've ridden is 27-32* on a night gravel ride... what I wore: clipless shoes with toe covers, med wool socks, thermal bib tights, MTB over-short, wool short sleeve base, cycling jacket, headband, light/windproof gloves.
I was still getting a bit sweaty on my back and my toes started to freeze after 1.5 hrs. The clipless shoes are ventilated and very thin, so a whole shoe cover would have been better. That outfit, for me, I think would get me down to 15* MTBing with MTB shoes instead, skull cap vs. headband and windproof thermals tights.