Mod has again published his "what to wear at what temps around here" list. It's a good guide and about the same as what I do.
You might want to print it out and put it with your gear. In your training log, note whether it was a good match and pretty soon you'll have your own personal perfect guide to what to wear.
"Yep, it's almost that time of year again. So I'm re-posted my clothing guide. I've been using this winter clothing guide for a long time and it's never let me down. Hopefully it will come in handy for some of you. The temp ranges are for off-road riding, if you're going out on the road dress for the next coldest range."
- MTB Omaha: Winter Clothing Guide (view on Google Sidewiki)