Everyone's got their own comfort zones. You'll get someone saying you need arm and leg warmers below 65ºF (really?), and you'll get people in Minnesota who ride in shorts in any weather above 20ºF.

This is something you have to figure out for yourself. My advice: thin layers do a good job. Avoid bulky items. Expect to be a bit cold when you start riding, knowing that your body heat will kick in later. If you are comfortable at the beginning of the ride, you'll soon be overheating when your body starts generating heat.