There are so many variations but the one I've picked seems to work well for me:
Per Chris Carmichael: cyclists should be in leg warmers or knee warmers for temps below 65 degrees F.
On my own I figure if I need leg warmers, I need arm warmers too! lol If it's cloudy, windy and/or damp I'll also wear long-fingered gloves to protect my little digits! You could also look into some light-weight jackets that fit in a jersey pocket (TE carries some) which would help keep your core warm when riding into the wind but can be tucked away when riding ahead of it.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"