I prefer knee warmers, which for my short legs go down to about mid-calf. If the day warms up, they are easy to remove during a long ride. That pretty much handles most days unless it gets below 40 deg F and I whimp out for my full length tights.
Above 60F I just wear my bib shorts. From 60F to 40F the bib shorts with PI leg warmers. And below 40F it the bib shorts, the PI leg warmers and a pair of jeans over everything.
I ride pretty regularly when it's well below freezing. I usually only need a set of Sporthill XC polyester/spandex pants over my shorts. I think they were meant for xc skiiing or something. Other than than, pedalling like crazy keeps my legs warm.
I have a pair of "cross country" running tights which as far as I can ascertain are the same as cycling tights except for the fact they have enough material in them to fit an adult human being and they are about $30 cheaper than "cycling" tights. They both have windproof fronts, breathable backs and are insulated.
For the really cold days (below 25f) I ride with "hotchillis" ski longjohns and wind proof bike pants, with extra agulated knee reinforced. also winter booties over the shoes. I also use my snow ski hement and goggles with a balaclava. don't go as fast, but confortable and make me appreciate it when I can shed all the extra clothes in the spring. Keep peddlin Melo