This doesn't directly address your question, but I thought I'd suggest it. On days when I think my knickers won't be enough but full tights would be too much, I wear my Shebeest "Pedal Pushers." At least on me (I'm on the short side), they hit a little bit lower than mid-calf. That still leaves some leg exposed, but if the temperatures warrant a knicker versus a full tight, I find that I'm only chilled for a little bit. Once I warm up, I'm fine.

http://www.teamestrogen.com/products.asp?pID=25847