Ender dragon mecnanics and nodes

I'm tyring to figure out the ender dragon mechanics to make a custom boss fight. I've spawned the dragon in a custom dimension but want to change which height he flies at (atm it's about 20 blocks above the surface and I want it to be right on the surface).
Do the nodes that the dragon follows/uses to pathfind generate upon it spawning or does it re generate them each time it paths to one? 
I understand that the nodes are generated about 20 blocks above the top block in the world at those coordinates. Can it pick any block to set the node above or are there some which don't count (i.e. maybe water)?