lol
I find that my knees are more comfortable covered up around 60F. Whether it is proper or not I do not care. Whatever a random doctor says matters not to me. What matters is how my legs feel.
My rough guide. It changes with the wind and how energetic I feel.
60F: knee warmers
50F: Leg warmers
40F: Lite tights
32F: Lite tights with leg warmers
25F: Heavyweight tights
10F: Heavyweight tights with long johns. I rarely ride at this temp.