There is a rule of thumb for runners dressing for a cold weather run: Dress as if it were about 10 to 20 degrees warmer than you would for just going out in whatever temp it is. Of course once exercising it will take a few minutes to warm up. I don't know what the "dress for" temp on the bike would be...because you have more of a windchill factor. And another thing that I always keep in mind is...if it's cold and I'm sweaty and then have to stop for a flat (or other mechanical issue)...that is not a good situation.
That being said, I like my cycling jackets. They're thin, and break the wind very well. So, I just layer unnderneath. If I realize after a few miles that I've overdressed...I can unzip to adjust my body temp.
Dan