Just take note when designing linear power supplies like this and you are concerned when your output will regulate or not. Take into consideration the line fluctuation. How low and up to how high the input voltage is, will affect what you get on your secondary.
If you are in a low line or brown-out condition, will your secondary be enough for the regulator's overhead voltage. Most 78xx ICs need at least 2 volts over the rated output voltage ALL THE TIME to keep the regulation.