You'll need to be on a ladder or scaffold of some sort to spray the top. That means shifting the ladder around as you proceed. It also means climbing up and down as you go -- on a higher step to reach across to spray the center, then dropping down a step as you spray closer to the edge and down another, then pulling the ladder away a distance as you spray over the curve.
Consider how long this might take, then take a weight
and hold it out in front of you for at least several times as long to get yourself in shape to do this. Believe me, you will be glad you did!
The vertical areas are easier in that respect, but one must remember that gravity rules. Runs happen. Especially if you are not real experienced. You may find yourself in a situation where the best approach is a bucket of lacquer thinner and a pile of old towels. I've been there and done that!
Clear coat is almost worse because it is clear and hard to see, especially while spraying. Going the bucket and towels route really messes things up at this point as both the clear coat and color coat come up. So you just let it go and attempt to correct it later. You can scrape runs to a point with a razor blade curved just right. Sand paper on a small block work. Some painters use a stone with water or oil.
Sometimes things just work out perfectly.
Regarding the amount of paint
, I always get at least double to prepare of such eventualities. It usually comes cheaper in gallons, if that's any consolation.