I stepped outside this morning at 10F, 1-2 mph winds. "Feels like 10F". It didn't feel extremely cold.
It's rare to have the winds this calm when it's cold, so just about any cold day includes a bunch of wind chill. That's what most people remember when they are trying to compare temperatures on different days.
I don't ride much below 40F. But even at 40, I dress differently if the winds are 2-4 mph vs 12-15 mph. The brief stops are much colder, climbs are colder, and it probably affects flat road riding, too.
Sunny
It's mostly cloudy here in the winter. My rule of thumb is "8 degrees warmer" on a bright sunny day. That usually works very well for picking layers to wear.