Working down the thermometer...

65 I add knee warmers
50 Switch to mid-weight bib tights
30 Switch to winter-weight bib tights
10 Add kneewarmers under the winter-weight bib tights

Always shorts in there because they're cheaper and easier to launder, a real issue as a four-seasons commuter. I have 12 pair of shorts at $40, but 12 pair of mid-weight tights at $125 and 12 pair of winter tights at $165 is a bit above my budget. Be cheaper to buy a car then.