Angie, I hear you. But it's so personal... and humidity/ wind related. It is about 18 here today...but SO MUCH COLDER than yesterday when it was sunny, humid, and supposedly 23 (but felt at least 30!). Today I am in jeans, raincoat, heavier cotton sweater, cami, sneakers. Yesterday I was going crazy in my white denim crops and a very thin voile cotton top! My sandals gave me blisters despite body glide -- such humidity, such swelling!
This is the variable that travel brings...so in the end I just have to hope. I will have camis, long sleeve full length base layers (in merino and Heattech and heavy thermal), cotton sweaters, cashmere sweater and shawl, merino sweater, wool blazer, scarves, hat, gloves...
At 20 without a lot of humidity or wind, I would typically feel comfortable in crops, a long sleeved shirt or a linen tee with cami, a light jacket or no jacket (depending on strength of sun). Sandals or maybe sneakers but my feet might be too hot in sneakers at that temp.
At 12 with rain and some wind (e.g. Vancouver or Seattle winter) I can manage just fine in: Booties, jeans, cami, merino or cashmere sweater, wool blazer, raincoat, scarf, gloves in case, hat in case. Make it 12 with sun and I could do a cotton sweater instead of wool or cashmere.
This assumes I am moving at all times. I almost never stay still...if I were staying still in those temps I would need a puffer coat for certain!! And at 10 degrees, I do often wear my light Northface puffer for a walk! But in that case, I wouldn't layer much under it -- just a regular top/ jeans.