Spring and autumn are the hardest months to dress for, because the morning and the afternoon can be ten degrees apart (sample guide).
Below 10 °C, wear a long-sleeve top and tights, and a wind shell if it is breezy. Between 10 and 15 °C, swap the tights for shorts and keep the long sleeve. Over 15 °C, a tee is enough.
The rule that works at every temperature: you should feel a little cold for the first kilometre. If you are warm standing at the door, you will be too hot by the end of the street.