Ceiling are just one flat surface....but your neck will tell you different.
In some charging sources...one is told to charge double than your normal wall rate for the chosen finish.
Remember to figure your layers. Is it really tin? So there will be a primer layer...if not already painted once before and then the other layers.
Sounds fun! I have a friend who has a real tin ceiling...never painted...silver thing...makes me nuts...have always offered to paint...and then at their lucky friend discount...has never happened.
Good luck
Debbie