I have a "soft" rule (i.e. flexible): under 65 degrees, cover the knees. I read it in a bike mag about 20 years ago. 65 is the starting point, and I'll drop lower if the sun is out and the wind light or out of the south. Really, the knees don't have any fat to protect them and get pretty cranky if they get cold. Sunday (the ride with the flat) I saw 61 degrees when I started before sun-up and 61 degrees when I finished three hours later. The light wind out of the north brought a cool front. Still, while the rest of the body reported in as quite comfortable, at around 35 miles my right calf started complaining. I don't think lack of tights caused the problem.
Yesterday my morning started at 55 degrees with a brisk east wind, so I pulled on the tights for the first time in months. Actually, the forecasted wind out of the east had moved to the south when I left the neighborhoods for the open road. Bummer! This ride goes mainly east-west, so I had a side-wind both directions (and, still under 60 degrees when I finished).
Two layers on my chest and the tights over my shorts gave me a comfortable two hour workout.
I have appropriate clothing for most cycling weather. I love it when I pick the right combination!