How thin should I mix the drywall compound ?

Pancake batter is about right. But on the thin side. Don't go 50/50 that would be way to thin. For orange peel the more pressure the better. So if you can get 50 that's where you should start. Also you will need a very fine spray tip. If you have the mud to heavy it's going to have a hard time getting out the tip and may spit the mud out. Orange peel, has the same look as a stipple paint. I'm only saying that to make sure we are talking about the same finish.